It is a vw or pegot (sp?) pump, (can tell by the gov spring setup). That would be Engine Rpm, as it has two reflective marks on the pump pully where the laser tach is pointing, and pump speed is half engine speed, therefore 2 marks would make the laser tach count twice, for each rotation bringing it to engine speed.
As far as the speed it turns, well with no pump action, no fuel, no load, for a few seconds... I bet you could turn it to 20000 rpm...
However we do spin VE pumps to 6000 on a regular basis in vw's. With 9mm heads.
1990 W250 non ic'd. Under the knife again.