Superb looking rifle I must say. Two comments: it appears the receiver was refinished after the markings on the left wall were engraved, which suggests some contact with Long Branch post-WWII. The "56" date on the barrel, if that is what that represents, must indicate either the year the barrel was made or the year the rifle was assembled.
My guess would be this was a presentation or someone's retirement gift to themselves or something along those lines. What is interesting is that either these MkI parts were still in stock at Long Branch as late as 1956, or else someone had squirrelled them away gradually over time and assembled this from those parts somewhere else: a LB worker, a military armourer, or anyone who had access to stocks of these parts in the 50s or 60s.
I haven't read the whole thread so if someone has made those points already no plagiarism intended!