I know of a Diana Mod.30 built to resemble a 98K Mauser, complete with a side-mounted sling. I presume it's a pre-WWII trainer. Does anyone know anything about these models? Value? I can't even find anything about them on Google so I suspect they're pretty rare.
You're right, it's a 30's vintage gun - underlever-cocking, single-shot, tap-loading - made with the same craftsmanship as other pre-war Diana's. This one is scarcer than some of the other 'trainers' such as Mars and Haenel. I sometimes see them sitting at gunshows with tags of $250 or up, though who knows if they ever sell. Occasionally they show up on Auctionarms or Gunbroker, which would give an idea of selling prices. Airguns done up in military-trainer style often get attention from collectors of military firearms as well as airgun folk.
BTW, you'd probably find a great number of non-common airguns won't bring up anything on Google!