Probably inspired by the Globe Co. Tokarev .303 conversions that were done here in the 60s. There may well have been spare parts available at that time and this was the result of someone's dissatisfaction with the recoil of a No.4. An old catalogue from the time would probably tell, if someone has one.
Pity they whacked the charger bridge off, as it makes the MkI sight rather pointless as it has nothing to rest on in the down position. Maybe you should get one of those 300/600 two position sights for it.
I don't think it's been reblued myself. I see that dirty brown colour of light rust, old oil and grease and congealed dirt. Spray it down with oven cleaner and let sit for 20 minutes then wash off in very hot water and dry with an oven or torch and then spray it down with your lubricant of choice. You'll need to strip it down first of course. The oven cleaner should get most of the crap out of the muzzle brake and the carbon you get by unscrewing the end and getting in there with a bronze brush. You'll need a metric wrench of course; I wouldn't use a Crescent.