There is Frankotte DR circa 1912 and chambers and bores look like .450-3", not most common 3 1/4", but 3". Case is sligtly tapered straight. There is sign on barrels 'lead bullet', but nitro proof is on barrels also. Nitro proof mark looks like more late Belgian proof, cause this mark is located just over sign Siemens Martin Steel.
Have you ever heard about 450-3" rounds?


Geno.