Just to add some further thoughts which I think extrapolate from your analysis, if whiskey was entered at a lower maximum proof than today, bearing in mind also the generally lower (than today) distilling out proofs, it would retain more flavor at younger ages than today. This seems to have catered for the public taste of the time. For those who wanted a lighter taste (but wanted to avoid drinking blends or gin), the product had to be aged longer, to convert those extra congeners. Thus, whiskey 10 years and over probably catered to this market but most of the straight whiskey market (pre-1920) was satisfied with 6-8 years as you said, it got the right "balance". As you noted, part of the older whiskey bracket may have been whiskey not aged for its full term in new charred wood. Maybe some of it was transferred to reused wood after, say, 6 or 10 years (something that can't be done today). This may have ensured that very old whiskey did not have an excessively tannic taste.
Today, since whiskey is entered and distilled out at higher proofs than in the 30's or before 1920, it must be aged longer to acquire a character considered acceptable by the market - longer than 3 years that is. (Jim Beam White is a big seller at 4 years but even that may be adjusted with some older whiskey and Beam White would in any case be considered I think a whiskey to mix with soda and other diluents by most). This may explain why, in general, we see higher age brackets in the market than before Prohibition and in the 30's, the 30's being however a special case for the reasons you indicated. (Also, there has been I think some influence on whiskey ages from the single malt market but how much is hard to say). I was thinking too of National Distillers and Schenley which would have been actively distilling since the early 30's (recall Mike they were distilling some whiskey before 1933 under permits) but I grant you the smaller concern was probably a different story. For someone in the position of the person being advised by Mr. Theisen he probably was looking to sources for product other than the big established concerns.
The above thoughts are restatements or interpretations of things you have stated here and you were the first that I know to note the effects of entering and distillation proof levels on the age of whiskey sold in former times (as well as the effects on the market of the differing bonding periods over the years).
To try to sum up, it makes sense to me that, as you said, before 1920 a heavy characterful whiskey wouldn't need more than 6-8 years to satisfy most of the market, and much of it did so at 4 years or even less, evidently, in some cases. Today, the lighter spirits being distilled and higher entry levels are resulting in bourbon needing more age to satisfy more people. Hence perhaps why so many premium whiskeys are receiving 8 years or more age, into the mid-teens and higher as we know. (Some of this may start to be affected by an apparent shortage of aged whiskey stocks, but this is arguably the case to date). Thanks again Mike.
P.S. I recall buying in the 1980's Henry McKenna bourbon which was available in our market then. I assume it was a Seagram product at the time. It was a good whiskey and may not have been that different from what was described by Theisen (even allowing for later changes in technology and entry level rules). It had I recall a nice dry taste but was not strongly marked by barrel tones. This kind of whiskey seems less popular today but maybe it will come back with the shortage of aged whiskey stocks. A bourbon can be flavorful and satisfying with just a few years in the barrel and that brand was a good example. I don't know if it tastes as good today, I haven't had it in many years.