These rifles were sporterised professionally in England by BSA (Birmingham Small Arms) in the period from the end of the 1940s through to the early 1970s, wth the bulks of them bing done in the middle 1950s and early 1960s.
They were available in several grades, all the way from 'just barely' cut-downs to very very nice semi-custom and even custom-quality grades. Yes, they were 'just old army rifles', but they were rebuilt professionally in one of the best plants in the world. When the work on them was finished, they were tested and approved under British law as new rifles. Many of them were even given new serial numbers in the BSA range.
ORIGINAL serial numbers for the 1917 rifle had only the number, anywhere from 1 to 7 digits with only Eddystone rifles running to all 7. Winchester and Remington each made about half a million, Eddystone (which was another Remington plant) made almost 1.3 million.
Whatever you do, friend, don't let ANYONE tell you that this is just another Bubba job. The work was done properly and what you were given is a damned fine rifle. I have one a couple of grades higher than yours (mostly cosmetic) which still has the original 1917 barrel on it. It shoots half an inch with its original 1959 Weaver K-4 scope in place.
A very good friend, recently deceased, spent 5 years playing with these old things. He ran through half a dozen of the BSAs and a Whitworth of Leeds, and there was not a single one that he did not get to shoot regular 3/4-inch groups (2 rounds from a dead-cold barrel: hunter's zero) and a couple shot well under that every time we took them out. Any rifle that ALWAYS shoots dead to point-of-aim with a dead cold barrel is a KEEPER for sure!
With the BSA-numbered bolt, as you have, headspace will be perfect. Don't waste your money on that. Get a set of bases, some rings and a decent scope onto the old girl and she just might amaze you. If she doesn't, then glass-bed her and she WILL amaze you.
You have a very FINE toy.
If you want to know a lot more about these fine rifles, come on over to the Milsurp forum and check some of the many threads about them. As for myself, I know one thing: I have 3 of these in .303 and one in .30-'06 and NONE of them are for sale!
BTW,the best ammo for these is the cheap stuff or handloads with FLATBASE bullets. These have Enfield 5-groove LH rifling (lasts half of forever) and it really prefers flatbase bullets.