4 corner cancellation on a KGVI stamp?

On stamps of Great Britain, they are called commercial overprints by those who specialize in them (see Commercial Overprint Society of Great Britain).

After the excise tax was lifted in 1971, remaining stocks of commercial overprints were permitted to be used as postage. Previously, postal use of commercial overprints was limited to certain cases.

I don't collect covers. I can only admire those that have been posted. I can probably pilfer a couple of pics and post them a little later...

I wasn't able to find a post-1970 example of Commercial Overprint used for postage, but here is an illegal example from 1967. Note that Commercial Overprints are not necessarily simple acronyms.

In the post for the OUS cover, I added the phrase "for postal use" in 2nd sentence of first paragraph. Others were already using Commercial Overprints in the 1850s for revenue usage. Since the 1d stamp duty was started in 1853, that should be the first year of any on-document example of revenue usage.

I did some more research. The duty was increased to 2d in 1920, and completely abolished starting 1Feb1972.

Underprints were offered by the Post Office in 1867, and discontinued in 1882. There was never a restriction on postal use of Underprints, as they were intended as a security measure for postage stamps rather than to preserve identification for fiscal use.

OUS began their postal use Commercial Overprints in 1859, and the Post Office withdrew permission for their postal use 10 years later. I interpret that as meaning postal use of Commercial Overprints were no longer legal starting around 1869. Hopefully, someone who knowns can confirm/correct.

Also bear in mind that my comments are strictly regarding Great Britain Commercial Overprints. Those of other nations/entities were not necessarily restricted from postal use. In this sense, sometimes Commercial Overprints are called Security Overprints, and basically served same purpose as Perfins.