The Pro Cal 512 appears to be a further development of the Calther 412, Brother's second electronic calculator. The Calther name was replaced by Pro Cal at a relatively early stage.
The 512 looks similar to the 412 and appears to be constructed with the same Mitsubishi M5900 series SSI logic family, combined with extensive discrete diode-transistor logic.
The main improvement in the Pro Cal 512 appears to be that the 512 can manipulate and display negative numbers directly. The Calther 412 handled negative numbers as their tens complement, so -1 was represented as 999999999, -2 as 999999998, -3 as 999999997 and so on. The user was expected to recognise and convert these numbers to their conventional representation.