There is a separate VT71 service manual that shows the two models with rectangular masks.
https://images.app.goo.gl/ppFCkTrScxoRpvjR8
Nope, no date
I admit I was wrong about the chassis pan and the holes. It seems that later production models didn't have them.
I'll check the database I've been maintaining of serial numbers, versions and production dates and see if that lends any clues.
Was just sent evidence of a "K" TS-4 revision. I thought that "J" was the last? It also has cream knobs but those might have come from a VT73 portable or a radio?
Very confusing and the factory using up odds and ends makes the most sense