that looks to me like its the cut down guide from a Sterling SMG spring. Not seen one on a STEN spring but the Mk5 cocking handle was to be used with a thin wrap around cover over the last coil but that's nothing like that.
That's possible. The recoil spring in the kit longer than regular STEN spring... There is also a CATCO tube in the kit, which is useless to me, but it came with the other parts.
The more I played with MK II, the more I like it: easy to dissassemble down to the pin level, easy to put it back, easy to find spare parts, the stock does not tumble at shooting, the rate of fire is ideal, the gun along with magazines can be packed into pistol case, not too heavy, etc. Lots of advantages on this model. No wonder it's counted as classic.