The A3PB41-600B-00 is a 44mm mushroom-head push button, 2-position momentary action, black actuator. The description on record is PB 2POS MOMTRY 44MM MSHRM BLK — that's a two-position momentary switch with a 44mm mushroom head in black.
Sourcing the A3PB41-600B-00 requires a quote against an RFQ.
The 44mm mushroom head is a standard emergency-stop or palm-actuated operator size. Two-position momentary means it returns to the rest state when released — no maintained latch. Typical panel cutout is 30mm or 22mm depending on the mounting adapter; verify the back-of-panel depth for your enclosure.