The 6 Groove barrels were made on Bren barrel lathes. If I'm remembering my readings right, there is no clear reason given for it. It was at the ramp up to Korea, so it seems likely to me that 1) they needed barrels, and 2) they had a open Bren lathe. There is a tiny "6" on the muzzle to denote a 6 groove barrel.

I've got pictures I will try to post. Apple doesn't let me send photos as an attachment now, which really makes it a PITA.

The two digit 0L79 is more "factory correct" than the 0L5. The 0L5 has been parkerized, has a replacement stock, and has the more modern slab style cocking piece, where 0L79 has the "button" style cocking piece, and the oil dipped finish. Also, the sight is incorrect for this early a rifle, as it's wearing a CmkIII backsight. Typical of some Canadianicon rebuilds, it doesn't have a /XX rebuild date on it.