Well, today I made some mesurements, bullet weight is 540 gr, it's double belted conical bullet. Molding stuff looks like original. Rate of twist is from 6 o'clock till 2 1/2 o'clock on 27.5" of barrels length.
Also made chamber cast and the case dimentions are matched to .577 Nitro Express 2.75" shown in Handloader's Manual of Cartrige Convertion, page 683.


Geno.