I have an SP1 carbine like this too. Though, it appears to be a 16" barrel with the 5.5" suppressor (total length about 17"). It looks like it is slid down over the barrel and threaded on. You can see threads around the crown and was able to slide thin piece of plastic up the back a couple of inches.
Has the 2 position stock and round hand guards, also has A1 sights, but the rear windage adjustment in knob (like A2) and not the plate style of A1. No markings on barrel either.
Ser # is SP 210xxx and as far as I know, I'm
original owner (no box, so not sure).
Bought it back in '86.