Variac manufacture year

Good day, everyone!
I've got a rather random question. A friend of mine and I have been trying to figure out when the General Radio Company's W5 MT Variac autotransformer was built. Here's what it looks like, if it helps:
**broken link removed**
I don't have the slightest clue when it was manufactured, but I would guess the '80s? I don't have much knowledge about variacs, so any help would be greatly appreciated.
Thanks in advance!
Der Strom

EDIT: I just noticed that the Variac in the picture is a W5 MT 3, which I know means it has three conductors in the cord (hot, neutral, and ground?). I am actually asking about the W5 MT, which only has two (hot and neutral).

The patent assignment to Gen Radio was published in 1960. Adjustable transformer with stabilized contact track - Patent 2949592

But, it could be newer than that.

Ken