Here is one source interesting for its stats:http://books.google.com/books?id=RnnNAA ... ky&f=false
It shows (and other evidence from this period and after does) that "bourbon" generally meant whiskey made mostly from corn, and rye whiskey was the term used to denote a straight whiskey made mostly from rye. One can see that rye in Kentucky was made in the 1860's to the tune of some 15% of bourbon production, certainly a minority but not inconsiderable and the report noted it was growing.
The two types were not viewed as interchangeable, but my point is simply that rye whiskey has been made in Kentucky for a very long time, by people that made bourbon too and they both come out of the same tradition. Old Crow was available in both bourbon and rye versions and both were made by a sour mash process based on earlier sources I've found although I can't put my finger on them now. Thanks Mike for your observations though which accord with my memory of what I read, at least for some brands.
Also, I was speaking from a broader historical standpoint: Kentucky bourbon and rye use the same ingredients and the flavour shifts as the relative proportions change, but it is all on a continuum, just as one bourbon which uses more rye than another will taste more rye-ish so to speak but is still bourbon. It is all straight whiskey and the regulatory code recognizes them as variants in effect.
This is not to say that a lot of rye whiskey, and clearly some bourbon, was made by a sweet mash process.
It gets confusing because as we've discussed here before, sour mash itself seemed to change meaning over time. I recall in that article by Charles Gallagher I posted some time back, and we know too from materials Mike Veach has referenced, that sour mashing originally meant a yeasting process apparently via slops or with their aid (i.e., no fresh yeast added as is always done today).