FWIW, overall I would agree with Lee Enfield & Prinz. I suspect it's a well travelled but genuine rifle. The only thing that bothers me about the photo's is that they seem to show that it has been re-numbered, & though that's not the end of the world I am also not quite sure if the butt socket markings might have been 'freshened up' by someone at some point. I know my impression may be an effect of the light, but look at the area just above & to the R of the FTR - there appears to me to be a faint '...44' present. Was it part of the original serial number (looks to be positioned a little high on the socket for that; or the original date???). This does leave me a little uneasy as to whether it has been 'got at' though I think it more likely than not is a genuine rifle.
It would be nice to have some more really meaty close ups of the butt socket, & perhaps of the front pad area taken from just below the wood line (ie looking slightly 'upwards'), to show the edge of the milled panel in which the front pad sits. It is generally just about visible on most rifles, & is a feature that in my experience is rarely copied by fakers as it takes too much skill to do & to get right! Another supporting feature of its authenticity would be to look at the front face of the body at 6 o'clock to see if there is a barrel indexing mark present. It may not be on the barrel itself as it could be a replacement, but the index mark should still be there on the body (receiver) regardless.
As has been mentioned the lack of a D6E, S S51 etc really doesn't mean a lot if it is an earlier & well travelled rifle.
ATB