This change was done after the sale of the Snow Trac line to Aktiv.
Some time in the 1969-1970 time frame. Both have good points and the not so good as well. Later machines, big wheel, are said to shed their tracks more easily, if the track line is tilted to far. The plus is, the tires run on the belt, thus protected by it. The drive sprockets are wider apart, and tend to shed the build up of snow better, and do not need the ice scrapers the two band machines need in refreeze conditions. There seems to be fewer parts like wheel guides, and grouser bars for these machines out there for sale at any given time..
The early machines pre Aktiv, are the two band tracks. The second bogie wheel is placed such that it takes every impact of bumps and obstructions. This was such an issue there was a year or two in the late 60's, the ST4B's received a cast hollow brass wheel in place of the rubber tire.. Personally I have had issues with two of those particular 2nd position bogies. The up side is, if the tracks are tight and the rest in sound condition, it is pretty damn hard to get one to walk out of it's tracks... Also there seems to be more used parts out there for the two band set up. I am thinking a big wheel machine with a two band track, might be the best of both, and yes they are out there... My 2 cents..
Hope this helps.
Regards, Kirk