Well that was unexpected. Apparently HP modified the shape of those little buttons sometime in the period 1979 to 1983. Who knew?
In the pic the longer one at left is from lowimpedance, off a HP 1744 scope (introduced 1979.) Matches buttons on the HP 8018A serial data gen (introduced 1978.)
The shorter one on the right is from the HP 5180A waveform recorder (introduced 1983.)
There's a slight change of the inside dimensions too, but the longer one does still fit the latter 5180A. It's just quite easy to pull off, and projects a little bit further than the rest of the buttons.
I'll measure the button lengths and add them here. Tomorrow.
Last laugh award goes to Nominalanimal.
Added dimensions (height, base square, mm):
Long (1979): 10.7, 5.85
Short (1983): 9.5, 5.7