Submitted by “Some Assembly Required” who says:

I’ve been enjoying your site for some time and recently remembered that I have an old MBTA system map in my basement. It came into my possession via a roommate over 20 years ago; I’m not sure how that person came to have it, but it probably wasn’t entirely legal. It’s a piece of metal (some sort of tin?) so I believe it was removed from a station.

Based on what is and isn’t on the map, I believe it dates to the early 1980s.

Transit Maps says:

You own this? JEALOUS.

(And your dating seems to be about right; definitely no later than 1982.)