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I've just received a pair of new 500/5 exhaust valves from a Morini dealer and there is just a groove for the collets not a smaller diameter parallel section of valve-stem as before and that I expect to see in 1980s Morini engines. So the collets I have don't fit these valves. I know there were two types of collet over the years. I've never seen the other type, does it have a raised bump internally to grip my stem groove, does anyone know? Would it be an earlier 350 type?
If so, is the exterior angle the same or do I need to source valve spring retainers to match?!

Many years ago, new valves I bought for my 500 rebuild had the small groove. There must have been matching collets and I don't recall needing a new or different version of the valve spring retainer. Before I reassembled the motor I did get an engineer to modify at least one new valve so that it matched the old valves but he did it very quickly and I wasn't confident enough to use it. (Someone else must have encountered these newer valves.)

edit: I looked through my stuff and have found the two different types of collets and two different valve spring retainers. I don't know if the retainers are interchangeable. I can email pics to you if you want to PM me.

Thank you and apologies for the delay finding your message. It appears the narrow groove valves are newer. Later collets fitted and have done the job, whilst keeping the same external 'conical' angle as the earlier ones, meaning only one type of valvespring retaining cap is needed