I'll have to double check details about the pin and chain. However, that elevating wheel with the finger grooves is one of the earlier style from WWII era. Also, Armorer is absolutely right on the B drawing number, definitely war period. When they dropped the letter sizes, they also added one or two digits to the drawing number. A 5, 50, 55, 6, 60, etc, depending on the original number. Early parts with four digit drawing numbers would have had a 50 or 55 or such added, while a longer number might just get a single digit. There was some rule to this I think, but I don't know exactly what determined a 5 or a 6 series added number. Might have been just sequential, just not sure.
Also, at some point they did eliminate the chain, and with it the loop that is integral to the yoke. No clue what year that was, but well after the war I believe. Same on the M3 T&E.