What is this (2)?

An answer to the question put by Ross the day before yesterday. Like Ross, S. has kept one of these in immaculate condition, down to the instruction notice. S. tells me he used this in the mid-sixties, when he was a technician at Flight Refuelling.

Didn't know which tag to choose: 'science', 'history', 'antiques', or 'boys', as in 'boys will be boys and they will MEASURE things ;-)