Mine's a Mk.1/2 FTR(F) rifle, with IIRC a matching bolt (I'll look when I get home). A-suffix to the serial number though, which is interesting. It's clearly been refinished again prior to conversion - the feed ramp is refinished and untouched by copper/brass, and the FTR markings are underneath the finish and weak.I've never seen what might be called a Sterling demonstration piece - a conversion with their factory marks.
Neither do any of the full conversions carry any sort of WD/MoD mark, as you'd expect if they had been done for trials.
However most of the full conversions I've seen were obviously on unused Mk1/2 upgraded rifles or - as with the one you've shown - rifles that were then refinished with pukka suncorite.
I'm guessing that a batch of the full conversions was done by a trade gunsmith at the order of Sterling, or as a commercial venture using the kits. Given the poor relations between Sterling and the MoD, I doubt the army touched any of the kits themselves.
The Charnwood rifles tend to be complete bitsas, whereas the Sterlings are usually based on a complete original rifle.