Well about the tag, I've looked at quite a lot of them to compare them and see how to date them by the tags, and by the early 80's already there would have
been both a Nato size and the British size system on the tag ("Size #"), but on yours it actually says the size in CM, which is different than the official systems.
It also seems like it would be pretty hard for the upper blank/faded bit to fit everything that seems to be missing compared to "normal" labels, item name, manufacturer
name, contract code, and also an NSN.
IIRC sometime during the 80's the 2-piece labels in windproof smocks were all superceded by 1-piece labels, on teh arctic windproof even in the late 70's already.
1 Thing I am not completely sure about are the chestpockets, I have a late 70's smock with them being slanted, and a mid-90's one with them being straight, so
sometime between they were changed but I never actually checked when as I just noticed this somehow haha.
However, I have an SASS smock in normal DDPM which does seem quite a bit like your smock but there's still a few differences.
About the buttons, as far as I know the 70's smocks already had large buttons as far as I can see on originals and old photos, sadly my own is missing pretty much all buttons,
not yet the Canadian buttons but regular 4-hole ones.
Here's some pics, the chestpockets are also slanted, the main label is also 2-piece and the lower part looks distinctly like yours but with a few more lines of stuff on it, but
in place of the size being on the upper label it's actually on a third little label piece below the larger pieces.
With my smock being made later it already has the Canadian buttons though.
I hope you don't see this as me trying to pick apart your smock, but I'm just going pretty deep into this as it's something I really want to know exactly about what it is and
from what company, and sadly can't find the post about the 1 other sparse smock like yours that I've ever seen which was on facebook and facebook is s*** when you
actually want to preserve information...
And also, I really really like your smock, even if it wasn't issue it's still cool as hell!