I've seen a lot of brunifixed (the textured looking parkerized finish) on 44 rifles, in fact I'd say on most I've seen that I could be certain hadn't been tinkered with later. It is difficult to be precise on when suncoriting first started exactly, as one could be looking at a 'late' rifle with original suncoriting or a slightly earlier rifle that got suncorited later on. I agree with Brian & Lee Enfield on this though - you have a peach. You can see by the crispness & depth of the S51 & other stamps that they've never been sanded or scraped.
Have a good look at the general appearance & 'feel' of the woodwork, coz that's how the woodwork on an unadulterated late war rifle should look.
Peter, can you shed any light on the date when suncoriting was initiated?