From the looks of it it is an ex-weedon (pre-ww2) upgraded pattern1914. From what you say about the rear sights and from what I think is written on the muzzle crown (parker hale ball burnished?) This is a Parker Hale regulated target rifle. Looks like there is still grooves at the muzzle so the barrel might well have considerable life in it and shoot well. Quite a few here in New Zealandalso as shooting 303british was very popular "in the Colonies". If it has no SA acceptance stamps then it was never in the SA army and hence surplused out, so its probably a private import. Its most likely that the gun was sold to PH as ww2 surplus after ww2 and bought by a SAer and imported for target work. Its fairly lightly used probably because once past 500yds an accurised no4s shoots better and hence no3s were not competitive. I can see some packing at the muzzle but I am un-sure that achieves anything as I am told they didnt respond to "improving" very much. In saying that I'll try it on my gun and see what happens. I do know ppl tried over-packing the no4 in a failed attempt to keep up with Omarks etc (which just didnt work) so this is probably just a home spun attempt, all interesting though!
The high serial number indicates its a * and I think made in 1917. However if you post here, Great War Forum (you will have to find the right forum) there is a guy who did massive research on these, telling me for instance that my gun was accepted into UKservice on the 7th July 1917 (wow).