I have just checked the boxes- it is marked ICI, blue box with red horizontal and vertical strip on the front. On the back- 20 Q M. Front says KYNOCH .577/500 no 2, 2 13/16" and 'metal base copper tubed, 440grs.
Thanks,
Charls
Charls,
Kynoch blue & red box = Nitro for Black. The 440 grain metal base copper tubed bullet was also only offered in Nitro for Black loadings in this caliber. Your cartridges contain Cordite or Axite (both smokeless), not black. The priming compound in these is corrosive, and the bores should be swabbed out with hot water or a water based solvent after shooting. Nitro for Black ammunition was specifically intended for use in black powder proof express rifles and was required to produce a peak pressure no higher than that of the normal black powder load. It usually actually produced less.
"20 Q M" is 20 October, 1938.
A few boxes of original factory ammo is really nice to have with a high condition British double rifle, especially in this caliber. The ammunition is valuable and old. I wouldn't shoot it. Components and dies are available to get your rifle shooting.