The title attribution poses another riddle: "PDP-1/B and miniature model railroad at University of Massachusetts".

The PDP-1B was the production prototype of the PDP-1 and there were only 2, we know of. The first one (white) was intended for LLNL, but the order got delayed and it went to BBN instead, the second one (blue) became the actual LLNL machine.

Update: Curiously, the this page [1] also mentions a "PDP-lB" [sic!]:

[1] https://www.cics.umass.edu/about/history-school-computer-sci...

Anyway, this seems to confirm the Amherst link already suggested by Steve Russell.