Yes Evert My Mk5 has an articulated cocking arm and my Mk4 has a solid arm and a piston head with circlip, except the circlip is different to the one shown. Mine is a minute circular item with a cut which touches when not in situ. You have to slip it over the end of the piston head shaft and push down and into the groove.
Also my Mk5 has a keyway head and piston as do the two Mk3s I am currently working on. So BSA swapped piston head styles around over the years of MK3,4 and 5 and must have been using up solid cocking arms for the Mk5 unless some one retro fitted one to this. If they did they would have had to lengthen the slot. I suppose the factory could have done it to use up some older stocks.
That piston head did not come from Scorpion pistol, the cylinder is wider.