I am inclined to agree wholly with Skippy, thread 4. Regardless of the number totals given, it is far from uncommon to see trials and test bed rifles upgraded to incorporate later modifications which promotes an earlier rifle to a higher spec but NOT wholly. We've seen it hundreds of times in our service, especially with early SA80 types used and over engraved or issued, used in trials and tests, withdrawn, slightly modified and reissued as something else......... Seen it plenty of times. Not uncommon. A lot of the stuff we had was marked in a similar way. Interesting and rare but common practice I'd say