> notice the .047 hum cap used with the three prong plug,
In 1972, 3-pin wall outlets were new-ish, not found in every venue.
This was a dilemma for g-amp makers. They could be sure many amps would be run on 3-2pin adapters, or have their ground pins busted off. The ground cap switch still got you a hum-reducing ground, if not a life-safety ground. (What's more important?)