Well, I'll stick my neck out.......
& say I think it's a good one. The appearance of the wood is quite typical of late issue WW2 walnut as is the manufacturer of it. I would concur with Peter's comments but would also state that I'm sure the forend stamps (even with the circle around them) are consistent with being original. When I bought the batch of T's in the 90's I also bought several sacks full of 'take off' wood. This varied from mint to well used & there were a number of forends identical to that shown (save for the serial number itself, of course). Re the S51 ----- yes it is a little atypical in its spacing, but they were all hand stamped don't forget, & the S & the 51 are all of the correct style.
The only thing that I don't like about it is the price.......
ATB
In about 1997 my pal & I bought about 140.....
No4T's that had all been gutted. Many had been stripped down to just barrelled actions; some were more or less complete (rifles only, no scopes or chests etc); although most were somewhere between the two. Some had both body pads on, some one & some both missing, but all were genuine & restorable with enough TLC & some spares. We gradually rebuilt them all over a period of years, selling off most (probably more than half to other UK dealers), but could afford to keep nice or rare examples for ourselves as they.....well....didn't owe us a lot.
IIRC we paid 10 pounds plus VAT for each rifle/barrelled action regardless of how complete it was......
We didn't complain.....In fact we didn't even haggle over the price. We just loaded them into my car pbq!
ATB