17mm indeed. They were also fitted to die cast K0's which actually had three of them fitted (two additional ones in the underside of the lower) crankcase), which provides a possible alternative source of them.
As an aside, there appears to be at least two variations of the 17mm head oil gallery plug - ones which are solid, and ones which are recessed internally. E302 and E1106 both have the solid type.
Cheers - Chris R.