The British .380 Revolver case is an exact copy of the .38 S&W.
The British .380 service load was originally a 200 grain lead pill, hence .38/200. The poms later went to a 178 grain FMJ.
The BSR was reportedly the result of S&W failing to deliver on a semi auto 9mm rifle design, they offered revolvers instead of a refund, being basically broke at the end of the depression.
Enfield couldn't supply enough guns during WWII, so the offer was accepted and in the end around three quarters of a million S&Ws were supplied.
Yours is in the correct serial range, its possible it was sent direct to a Commonwealth country and never issued, hence no acceptance marks.
Commercial 145 grain ammo or surplus Commonwealth ammo will be safe in your gun. Any hand load in a reloading manual, ditto. Apart from some the .38 S&W Ruger Speed Six variant, its probably the strongest .38 S&W made.
It lacks a positive internal hammer block, so treat it as a 5 shooter, leaving the chamber under the hammer empty.
Edit to add. Lanyard ring is correct, that was part of the spec. the Diamond grips appear to be pre-1936 commercial style.