John's opinion is highly valued but here is what I think.
Monongahela whiskey originally came in new (white) and not long after, aged varieties, even before it was shipped very far.
I have read enough 1800's references to red or "old" Monongahela whiskey (including Herman Melville's in Moby Dick) to suggest this was so even before the whiskey was bottled.
This whiskey was somewhat, in its aged form, like Old Overholt today. Overholt still uses more than 60% rye.
Maryland rye generally was redder in colour and while often blended (e.g., Melrose, which was a blend of straight ryes) some was sold straight and originally made in Baltimore, Loreley, Havre de Grace, Cockeysville and many other places (see again Bready's article, also John's fine American Whiskey Pages at ellenjaye.com).
I think the local Maryland manufacture, dating from the 1800's, imparted a rich, sweet, spicy quality. It may have come from malting some of the rye used in the mash. I have had rye beers that are reddish and spicy and a distilled rye beer would have been too although the colour would have to come from the casks. Maybe it was Frank Wight's yeasts that did the trick (which perhaps had a regional characteristic). Whatever it was, this seemed a regional style and that fruity quality is remembered, correctly, on the current Heaven Hill web site, in respect of Pikesville rye. Except, I do not find Pikesville particularly fruity-tasting and I am not sure it ever was. But that doesn't matter, someone at Heaven Hill knows that, in general, Maryland rye was fruity-tasting and he is right.
The aged version of Monongahela survived after Prohibition and in somewhat altered form to this day, sometimes with more corn than was used in the past. Examples are Old Overholt, Wild Turkey Rye, Thomas Handy rye. I think Kentucky rye simply used more corn than Pennsylvania and Maryland ryes did although that did not fundamentally change the palate of American rye whiskey in my view. (I think too recipes must have varied even for the Mon distilleries discussed by John in his Whiskey Pages). In Lancaster County, PA in 1809, Samuel M'Harry indicated familiarity with all variations of rye and corn mashbills.
E.g. at John's I tasted his pre-Pro (entirely made and marketed before 1920) Old Overholt. It was superb and better than anything available today but clearly linked in my view to the modern whiskies mentioned. And that bottled Overholt was, I believe, similar in character to most aged Monongahela rye made 30 and 50 years earlier. I can't prove that but I believe it. I also tasted a circa-1900 Baltimore rye that was richly fruity - the classic Baltimore taste. So these were the two poles of pre-Prohibition rye whiskey. As for rye made before Prohibition in Kentucky, probably it followed more the Monongahela style. I remember a letter Mike Veach posted where a bourbon distiller in the 1800's deprecated the Maryland distillers. So probably the Maryland style did not appeal to Kentucky palates. Also, rye whiskey would have come into Kentucky from Pennsylvania, and Virginia to a degree - Maryland would have had little influence on Kentucky distillation practice and I'll give you another example of that. Kentucky practiced sour mashing and Maryland did not (per Bready again or Carson in his Social History of Bourbon - one of them wrote this).
The Maryland reddish, sweetish, spicy, "juniper" style also (I believe) survived Prohibition and the vatting and blending that happened during Prohibition. Frank L. Wight (who started in distilling before Prohibition) and others kept it going. Today in my opinion, nothing really resembles that style. The closest is Three Grain from Kittling Ridge in Ontario which has a sherry-influenced taste (from finishing in sherry casks) and it reminds me of the straight and blended Maryland whiskies I have tried but is much lighter and less intense. Mount Vernon, as tasted at John's, reminds me of it to a degree but John is right, its profile became (even in the 30's) more similar to Western Pennsylvania whisky. Mount Vernon was originally established by a family from Philadelphia, so from the beginning its origins were perhaps mixed although I think the earliest examples were probably very Maryland in style.
Whiskeys I believe were broadly in the classic Maryland style (whether blended, blended straight or all-straight) were Sherbrook, Sherwood, Ruxton, Melrose, Melvale, and the aforesaid 1900 Baltimore rye tasted at John's. I am not sure about BPR. I have not tasted most of these but based on descriptions by others or various reading (e.g., the booklet I have on the history of the Melrose distillery which speaks of a blending agent used to marry 5 straight ryes - that had to be a sherry or other like-concentrate), I think Baltimore rye had in general this fruity, estery quality. The statement on the Heaven Hill site, derived from industry insiders, is further proof. And we see that the Sherbrook straight rye from the mini (see the taste note in the first post in this thread) meets that description to a t. Case closed.
If you take that Three Grain, add a dash of Pikesville to pick up the rye character, and add some sweet rich bourbon to impart new barrel character, that would resemble I think that Sherbrook or some of the classic straight or blended Maryland whiskies.
Dave's 10 year old Wight-distilled Sherbrook is (I suggest) a definitive form of Maryland straight rye whiskey. Dave, can you give a taste note of it? I project that it tastes sweet, spicy, spearmint-like with juniper tangs. I don't really see it as similar to Maryland Pikesville rye. I think the latter was atypical Maryland and was more the drier, austere, "toasted"/varnish style like aged Monongahela was.
It is important again to note the reddish sweet taste of Maryland whiskey was not always the result of blending. Wight made many whiskeys (i.e., at different distilleries - see Bready again) that had that character that were sold straight. Many other Maryland whiskies were sold straight. If anything, the matter was the reverse: the blenders sought to emulate the best qualities of that straight Maryland rye.
Gary
P.S. In the Melrose booklet I mentioned, there are numerous straight ryes in the product list at the end of the book. In total there are 4 blends - two of which are blends only of straight, or straight rye, whiskeys - and 3 straight ryes (one bonded). I think most companies probably did similar. A gradation of quality would have existed. E.g., whoever distributed Sherbrook probably had different blends out too. The whiskeys would have tasted similar but with different intensities of flavour.