I started this thread while researching the 360EX Watson double now in hand and with its own thread here. That rifle arrived with vintage ammo collected by its previous owner -- almost all of that ammo was nitro-for-black LRN copper tubed.

Watsons has since supplied their ledger entry for #13566. It was sold in 1906. The rifles immediately before and after mine were 450/400 3.25" case, with one including "cordite" in the entry and the other (like mine) with no mention of cordite.

I am approaching my rifle assuming it was not nitro proofed and working on NfB loads.

Thanks again for everyone's help here.