Congratulations on your find. The swivel under the upper guard is another Trials rifle feature not found on production No. 4 rifles. Given her age, she is not too bad. Trials body, matching serial numbered Trials 1930 dated barrel, proper and rare early wasp waisted MK. I foresight protector, rare upper swivel (stacking swivel I believe), MK. I button cocking piece, Enfield stamped bolt head. Don't worry about a sniper swivel in front of the magazine as that was a 1945 period add-on. Yours has the Enfield examiner's stamp on top of the front scope pad which shows that it was one of the first 1,403 No. 4 rifles converted to snipers by Enfield in 1941-1942 and not the 1942-1946 conversions by Holland and Holland, so it would NOT have has the "S51" stamp on the underside of the butt.
The likely explanation for the A suffix instead of prefix on the bolt handle is that non-standard rifles sometimes later had an "A" suffix mark applied AFTER the serial number to show that these parts were non-standard and were not interchangeable with normal service rifles.
If interested, have for sale a spare used walnut No. 4 MK. I forestock from 1941 production (SN 25077) with the proper notch for the magazine cut-off. I bought it to use to "restore" my Trails No. 4 sniper but decided to leave mine alone as it is matching in-service with a 1945 No. 32 MK. III scope, and forestock (no notch) numbered to the rifle, likely all fitted during the Factory Thorough Repair and it is exactly as it left the BritishArmy, There are 1963 stores tags matching the scope and rifle in the No. 15 chest with the set. http://captainstevens.com/militaria-...ated-firearms/