There are 2 variants of U174 plug. As I recall most if not all the Robbies I ever flew had the US(?) version which has a chrome coloured pin. The Enstrom's here seem to have the UK version which is brass coloured with a pointy end. Not compatible with a robbie.
At least one aircraft I flew didn't follow this rule. Cant remember which.
I've got adapters for both.
Total pain in the arse.