This is a Short, Magazine Lee-Enfield rifle Mark III, built by London Small Arms in 1918.
London Small Arms was a consortium of the "best" London makers, built rifles only until the end of WWI. The Company was wrapped up in the 1920s. During the Great War, they built only 5% of British-made SMLE rifles. They built NO rifles during the Second War, although members of the consortium did build sniping equipments, etc.
Since then, this has been converted into rather a nice long-range Target rifle; the rear aperture sight is ample proof of that, as is the Comercial BSA barrel.
The marking "MARTIN ADJUSTED" tells us that it has been through the Glasgow shop of Alex Martin (inventor of the "Martin Ribless" Best Quality double shotgun), where the bedding and all fitments were thoroughly adjusted an the rifle "regulated".
When I started shooting, this was the preferred rifle for 1200-yard iron-sight Service Rifle competitions. The Number 4 worked very well out to 600 or so, even could beat out SMLEs, but the SMLEs ruled the roost from 800 to 1200 and there was no way around that fact.
It was (and may still be) a VERY good Lee-Enfield.
I shot with Alex Martin's nephew for 25 years. This is a rifle I would really like to own, but I am afraid that I don't have the money.
I would say 400 is on the cheap side, unless somebody just wants a wallhanger, but one THIS good deserves to be shot.