Another idea occurs to me about the reason for the 1974 distillation date of all the Hirsch Michter's.
Pennco was unquestionably a bulk whiskey producer. For example, it apparently produced rye whiskey for the Overholt label, and other rye labels, at various times.
No doubt it also made straight whiskey sold in bulk for blending - American whiskey (the blended product) was a big seller in the era in question (it still is in some parts of the country, e.g., I saw a lot of Kessler's on the shelves in Santa Rosa recently).
Maybe someone had contracted in or about 1974 to buy 1974 distillate from Pennco for a private brand. If this is so, this would explain why that year's production was in those barrels and nothing else - that is what they bought. Also, since the Hirsch labels identify the contents as bourbon, the purchaser (in this hypothesis) would have stipulated for bourbon so the mash used would have been a bourbon mash. Bearing in mind how close the regular Michter's-label whiskey was to a bourbon mash, that would not have been hard to do (throw a little extra corn in there).
For some reason unless the original buyer wanted old whiskey (which is possible), it or he did not take delivery until 1989 or by then had sold the whiskey to the people who did ultimately take delivery. The consignee removed the whiskey to Kentucky later to be tanked (for the 16 year old) or further matured in wood until it was 20 years old and bottled. The 20 year old was never tanked. Either way, it was stored in Pennsylvania, clearly, for the first 15 years, but then was aged as Jeff said in Kentucky.