If it's another Michell part it won't be from the Hydraulic reference although a focus one might be a possible. They're not that expensive (or that good) so one for spares or repair might be worth a look but it still won't be cheap and if it's a non stsrter it'll have to be offloaded. It's worth a look but I think the helmet is worth further investigation. Interesting thing about the riot helmet thing though; even though the chin cup kight be a match the police in the UK didn't really use this kind of equipment in the 1970s. Widespread use of this kind of thing wasn't in use until mid 80s onwards although some were being used in the 1981 Brixton riots so the correct model of very early riot helmet will be very rare.