Couple of things I saw the S is there its just very hard to make out, pic #29 there are vice marks on the inside of the bracket I am pretty sure a real armourer would not use steel jaws on a bracket.
Have a good look at the crown there are 2 distinct dings that are at @8 & 10 o'clock a re-crown straight away bore looks dirty.
I think some enterprising bubba has tried to get those screws out without sweating the solder as thats how the Brits did the screws, not sure on all that crappy engraved serial on the bolt handle it should have been linished smooth then re stamped or engraved, Butt is probably a replacement as there is no S 51.
I would wait till the last 30 seconds and see where the bids at before you have a crack at it , think the last L42 I saw here with complete kit even the plastic brush and bottle had $15,000.00 on it not sure if it sold.