As I say I couldn't see the rivets. If there are three on the outside and it has the later plastic cradle liner that makes it M56/66 sometimes called "transitional and it is an early shell with the rivets "blinded" and six new fixing nipples spot welelded to the inside. This is probably what gives the impression that you expect to see six on the outside but the rivet heads on the outside are not related to the fixing nipples on the inside. This is due to the use of earlier shells which had already been bored for the three rivets when the new liner came into use. Spot welding the nipples does not cause the heat issues affecting the ballistic quality of the steel in the same way that welding up the pre-bored holes would do. Hence the "blind" rivet heads on the outside. This ties in with the idea of this being one of the earliest plastic cradle helmets.
Regards
Mark
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