The seller has misunderstood something I fear.............
The L12 designation was the designation of the sub calibre KIT and not the rifle. There was NEVER a designation of RIFLE, .22" L12. THAT L12 rifle designation WAS THE PRESERVE OF A SPECIALLY ADAPTED SHOTGUN. And what I see doesn't look much like a specially adapted shotgun..........! The L12 kit was KIT, conversion 0.22" calibre, L12A1 for 7.62mm rifle..
That is only my view based on nothing more than my bit-part experience, the EMER, the parts list and the L12 user handbook.
We did have rifles with the kit permanently fitted, caused by the .22" barrel bulging and jamming solid in the 7.62 bore. These were still on charge as a rifle but relegated to indoor training (or out door poaching on the training areas.....) but with a red band around the handguards and all unnecessary parts, such as gas parts and return springs stripped from them. When this occurred, the now .22" rifle was retained IF THE REQUIREMENT EXISTS and a new rifle issued as a replacement. Alas, when the L1A1's were called in for replacement by the L85, these .22" rifles seemed to languish in the backs of armouries or Armourers shops until someone decided what to do with them.