This looks like surface mail (no airmail label), and as it's from HMSO it's almost certainly printed papers rate for some Government publication or other. Printed papers had to be sent open to inspection so wrappers definitely qualified.
Not sure which
printed papers rate it was -- if it was a newspaper, which wrappers typically were for, then it would have qualified for reduced rate. On the other hand, wrappers of reduced rate items were supposed to be marked to say they were reduced rate, and this one isn't. So maybe normal rate for some printed brochure or whatever?
Normal rate second step (2-4oz) was 6d 1966-71, reduced rate third step (4-8oz) was 6d 1968-71, so unfortunately that doesn't pin it down. (Yes, going by the rates page
Another nice item, of course!