Darn good Q Swarfy. Hadn't blown pic up to spot lack of threads for big cb cover, just assumed it had that feature . . . Hmm. Part number is a later one than I'm up to speed on, but as far as I know, no Comp mag didn't have a screw on cover, with breather. Someone here will recognise the number though, I'm sure, and throw more light on things.
There is a large number of part numbers for the cb end housing, different versions, different years. The e-bay seller triumphbsaparts was listing what you have as 455777 C, and also selling 459279. Lucas parts lists (from the earlier years up to '57) list at least another 4 versions (I gave up looking after a bit as you go cross-eyed) with different numbers, for manual, fixed, different end covers, opposite handed AR cable entry, etc etc. Later, there were other variants too, such as the fixed camring Competiton end housing, with the AR entry lug cast but not drilled and finished. Nightmare!
Were some standard mags 'square' with the bigger threaded holes in the body and clip-on cb covers? Maybe, but haven't seen one. If so, then maybe it's 'right'. It would go on a C or R version of K1, 2, VF - or some of the other single cyl mags like N1 (C and R versions with ringcams). But it would look wrong, and one fastener would have to be a pillar and clip obviously, not one of those handsome double-cross-drilled deep-hex screws for wire-locking.
Or are there replicas being made which don't have the details right . . . I'd say probably, yes.
My own most 'modern' standard K2F mag is 1966 stamped, on a '66 Notrun, and that has a rectangular end housing and 2BA screws, with the normal later big screw-on cover and baffle.
Others on my own magneto bikes are standard rectangular with clip-on end cover, std with ali dome, std with home-made ali cover - all 2BA threads - and one 'normal' K2FC - square end, 1/4BSF threads.
Said it was a complicated area, with loads of potential pitfalls for the unwary. It is.
