Thanks Tim, for the photos, my guess the spring cap was the reason for this defect. They did not get collets with correct taper for the spring cap, or the stem groove did not match the collet shape - or the spring cap was too soft brass or alu. The single groove types come in several different types, see my photo. But I would not pick them as that type of collets leave a gap and squeeze the stem - unlike the three groove precision No. 22 MK8-H collets with no gap. So their perfect taper spreads all loads evenly and should go well with alu caps. Anyway, either types are produced and installed in millions of engines without problems. In your case I can imagine that the collets digged into the soft spring cap at the top edge to form a substantial ridge that prevented easy disassembly - my theory. The printed sample of collets is in a TRW book , more than 20 forms in there - but definitely no combo with a tiny wire clip.
Vic
Vic