Your rifle started life as a WWI P17 (Model of 1917) made in USA - either by Remington, Eddystone or Winchester factories. BSA bought gazillion of them when they were surplused - ran through their heat treat ovens to anneal the receivers - milled off the rear sight ears - drilled and tapped for scope bases and aperture sight, installed their round plug into the "duck pond" and stamped their trademark there. Yours is the first that I have seen with three scope base holes on rear bridge - somebody added one?? I have at least a couple of those BSA conversions here - as previously posted on CGN by others, much of the "hard work" to convert them to a hunting rifle is already done. I read somewhere that BSA re-heat treated the receivers after they were finished the modifications - so the receiver would have 1950's heat treat - not from 19-teens. I would think for an elder one like that, the bore condition is really important about value or what to do with it. There were at least three contractors during WWII rebuilds of those rifle - High Standard (HS), Johnson Automatic (JA) and one other - that made replacement barrels - many of those "new made" barrels were surplused after WWII - will screw on exactly to that receiver, if the existing bore is toast.
Of the ones here. BSA did a thorough job to polish off the original maker's marks from the receiver - they stamped on their own serial number to the receiver and to the bolt. The various small parts like safety lever, trigger, cocking piece, etc. might still have the original makers marks, but more than likely, they are all mixed up in the same rifle. Was a thing in those USA WWI rifles - virtually every part except coil springs and screws will likely have a "eagle head" (military inspector acceptance mark) and a letter - "E", "W", or "R" to show which factory (or sub-contractor) made the part.
A Enfield P17 - as per USA Instructions from WWI - was designed as a "controlled round feed" rifle - but was also made with ability to "single feed" - so is reasonably desirable by many for conversion to hunting rifles. It is a "#### on close" rifle, though - some feel that is correct - some feel that is "wrong". My Dad carried a P17 since he "farmer sporterized" his, in 1948.
Thinking - that third hole on receiver bridge - might be that funky BA thread hole that accepted the screw that held the flat spring for the rear sight assembly - the scope base holes added by BSA would be the front and rear ones. I think Parker Hale was a common scope base and ring maker that had been used in those days on those rifles.
That I know of, BSA sold their P17 conversions in at least five "grades" - "A", "B", "C", "D" and "E" - not sure that I have ever seen a listing of what, exactly, was different, one grade to another. They also made some with "new" barrels - is one here - either a "D" or an "E" grade - chambered in 270 Win.
