Interestinger and interestinger.
I still plump for the Spitfire angle. But both this and the B17 gunner option should be the easiest to disprove, at least. There are still many of those gallant chaps around. Some of you will know them - could you ask them, please? If the phrase was in common usage, it would surely feature in the odd battle report.
If none of the men directly involved can remember it, then I would think that would be proof enough that it has nothing to do with machinegun belts.