Bonedomes and Chipmunks
I think the Mk 4 in the Chipmunk problem may have been missunderstood. As I recall if your Chippy harness was slack or you were a bit big during negative G your bonce protection rested on the canopy a bit. The Mk 4 had the same visor arrangement as the Mk 1. the plastic lump on a slidy thing. The plastic lump had a ridge at the top and in that state all the weight was resting a a small area of canopy making nice little slots in it. The Mk 1 was modded by filing bits off to make it a bit lower and wider so the weight was OK but then of course it was non standard and so could not be used elswehere.
It was a long time ago and my memory has been faulty elsewhere so humble sorrow if in error