The break between the 1rst stage body and the swivel cap along with the metal trim piece under the yoke clearly makes it a Mk 5 and I agree it is probably early to mid 70's vintage.
The bulgy looking swivel cap is for the old style SPEC system. There are two ports on it with sintered iron filters that were intended to keep most of the silicone grease inside the ambient chamber. If you look in the bottom of the hex holes in the ports, you will see the filter - or at least dirty looking silicone that has oozed out.
SP moved away from this design to a simpler design with several very small holes (about 1/16") and eventually to a system with a rubber boot covering slightly larger holes. The problem with this earliest design is that it still leaks some silicone so it needs to be topped off periodically. The silicone available most places today is not as thick and ooozes out even more than the stuff SP used which complicates the problem. Also, if not packed properly, water can get trapped next to metal parts in the ambient chamber and cause some very nasty corrosion.
My advice if you plan to dive it is to have it serviced and if possible find a different swivel cap for it. Failing that, leave it silicone free and leave the plugs out to allow water to circulate and to make it easier to rinse. That of course means it would then not be so great in terms of a cold water reg.
