Over the years I have often seen these non-standard rifles being sold to the general shooting public as "rare" variations of the rifles. The Gibbs 2A1 being one of them. What the builders/importers do not tell you is that the rifles when received as surplus from overseas often has a number of rifles that have bulged barrels at the muzzles from mud or debris being in the barrels as they are shot. In order to recover the loss of the rifles the importers will either sell the rifles as scrap for parts or modify the rifles into a short version of the standard rifles. A lot of the initial production commercial M1 tankers are short rifles built with original M1 barrels that were originally bulged and then shortened to 16 inches. I have also seen the same for semi-auto version of the M16rifle barrel shorten to 16" inches. In a lot of cases the M16A1 rifle bores are in excellent condition it is just the front end of the muzzle is enlarged from the debris in the muzzle while being shot. I suspect the same is true from the large number of No4 MKI or MK2 rifles that were imported into the US from overseas. They sell better as a No 5 MKI Jungle Carbine.
Cheers
--fjruple