GMCS,
No clue as to maker. You are correct that it is 6.5x58R S&S. The 16 in a circle means it has 65mm( 2 1/2-2 9/16")16 ga. chambers. The crown W means that barrel is choked, but amount of constriction not given. The crown G means that barrel was proofed for a single projectile(bullet). The crown U is the mark for a View proof, performed after definitive proof, which amounts to a detailed inspection, including verification of dimensions. It was proofed in September 1926( 9,26). The numbers under the date are the ledger number. The ledger number, along with "Nitro" written in script, confirms the gun was proofed( and likely made in or near) in Zella-Mehlis rather than Suhl, as you thought. I don't know of any current source of commercial ammo. This cartridge is largely a hand loading exercise and a fairly complicated one at that. Cases are difficult to make and bullets are usually smaller than available .264" diameter. If you can find custom loaded ammo, it is likely to be expensive, as would cases.
Mike
