That is interesting, he must be a great guy to talk to. I seem to recall that on http://www.straightbourbon.com
some time ago, in promo materials a member posted from Michter's from the 1980's, it was stated that distilling-out proof was not far under 160 (maybe 156). It sounds like the 20 year old rye when distilled in the 1960's, may have been distilled out a lower proof than that. Maybe that was the practice then, or for its rye whiskey. Or maybe anything under 160 was regarded as low.
The mashbill described is the same as for Michter's Original Sour Mash as described in Michael Jackson's late 1980's World Guide to Whiskey except the latter had 50% rye and 38% corn. Jackson wrote that Michter's used to supply straight rye at times for the Overholt and other brands. So this 20 year old stock must have been kept for that time atypically.