Mickey, John....
Nearly all if not ALL non-portable belt-drive Kenmores from the mid-70s (let's say 1975) forward had a standard 4-minute drain, regardless of capacity. This is because most timers where shared between the two capacities, and a 4-minute is necessary in a super capacity model. I believe the same to be true for Whirlpools, at least from the late 70s onward.
In addition to this, many of the early quick-disconnect timers which originally were cammed to have a 2-minute drain were revised to have a 4-minute in replacement timers (thus changing the timer part number). John - you may be familiar with common replacement timers 660618 and 660651, the ancestors of whice were used in the early 1970s extensively in 1972 and 1973 'plastic top' machines. These replacement timers have 4 minute drains and shorter first spins but the original timers they replace do not. The original timer 365996, for example, has a two minute drain, and was even standard in the 1973 mega-Roto-Swirl equipped 18lb. model (110.7324500) but it subs to the 660618 with the more intelligent 4-minute drain. This timer is also the spec for a 1972 24-inch machine, so it too has the agonizing 4-minute drain, 2.5 minutes of which are probably used up sitting there doing nothing.
Belt-drive Kenmores from the mid-70s until 1986 used only a handful of different timers. Most used what is now timer 381860, 378133, or 660693. Models encompass 24-inch, 29-inch standard, and the most popular large/super capacity variants. My 1976 Kenmore 60 (the Green Beast) and my 1976 Kenmore 70 are both standard capacity machines, but they use the ultra-common 381860 timer which was used extensively in large capacity machines as well as in standard 29-inch and 24-inch models. This is the timer which has Normal, P Press, Knit/Delicate and Pre-wash (no soak). These machines thus have a 4-minute drain. The 378133 adds a pre-soak and is capable of timer set temperatures, and the 660693 has the second rinse option and auto-soak advance. The original timers and the revised replacements all have 4-minute drains.
Maybe I'm looking at this from the wrong perspective, but as it looks to me, a 2-minute drain was standard until there was some history for a few years with 18lb. models. Drain periods were lengthened to accomodate these machines, and as manufacturing commonality became more commonplace at the same time, other machines that did not necessarily need a 4-minute drain got it by default due to use of the timers that were used in the larger capacity models.
Gordon