Would have been built between 1914 (introduction, according to my Standard Catalog), and 1915-1917 or so to have been carried & used in WWI.

No British proofs visible in the photos sent.
So, roughly 7000 of this Triple Lock variant?
Anybody care to venture a guess on a price range, with holster?
Holster well used, but looks sound in the photo.
Standard military brown full-flap with post-strap retention.

Buddy has nobody to pass it onto & no idea what it's worth in the current market.
I may make him an offer & don't want to cheat him.

Guess I shoulda mentioned this one's a .455 Mark II HE 1st Model.
Denis