OK, so I am still banging on about the variety of sizes of screws all over the airplane. The lower wing to fuselage panels are somewhat over engineered with a large number of fixings which is just as well considering the number which are the wrong size or stripped thread. I am replacing with the correct size but have found an aviation source for M4 x 16 raised slotted stainless difficult to find.
Adventures with a Bölkow 207