Strange RAF 'Engine' Cap Badge

[SAS1]

Does anyone have a positive I'D on this cap badge? It's a standard Kings Crown RAF brass die struck cap badge but with a brass 'engine' placed over the front.

The badge is mentioned in Margaret Nobbs 'Nobbs Notes' with a question mark, and I've not positively identified it either. The first one I saw appeared on ebay and went for something ridiculous like £180.00. I wasn't going to pay that, but believe it or not shortly after this one appeared on ebay Canada and I got it for about £20.00. They are the only two I've seen in almost 40 years collecting.

The 'engine' (or is it a rose?) is as well made as the rest of the badge but there is no sign of how it's attached. Id think its soldered/brazed. From the back you can see the 'RAF' lettering so appears to be an attachment on a standard issue badge.

Is it connected to apprentices? Maybe the Polish Air Force? Pre-war, post-war?

Any ideas?