Try tracing the origination of the shirt.

Where was it bought? Does it have a label that indicates if it was a mass-produced shirt, or if it was screen printed by a small shop (which would have a generic tag such as hanes these days).

Many small t-shirt shops back then would hire local artists to do drawings, and then do print runs of them. So could be Nicolson is a screen printing shop, not the artist.

But you're best bet is to try and figure out a region the shirt was created in, and start looking there.

no ideas for ya, but very cool shirt!!!