GB Postal Marking Help

Any ideas what the diamond marking means on these two airmail aerogrammes? I am thinking "foreign post". Any help?

These diamond marks indicate that the item was counted in the British Post Office's census of mail volumes. #

The markings were applied to incoming and inland mail as well as items being sent overseas.

The marks I've seen before have been from dates in October (from many different years) but it's interesting that one of these two was from June.

I believe the practice ended some time in the 1980s.

nigelc: Thank you for the help. Will note that information on the display page with the covers in the my GB volumes.

The census diamonds were applied by putting the mail through the cancelling machines on a separate run -- the cancelling bars/slogan were removed (or were supposed to be -- sometimes they forgot/didn't bother) and the datestamp portion replaced with this diamond mark. The reason for it being done that way was because the (single impression) cancelling machines had a counter of strikes made, so this was a convenient way to count up the items of mail.

Interesting. Appreciate the ID.
Another shape that appear as Ettiquettes in Back of book collections,
can be the "AO in a diamond"

Country of origin still unknown by me.

Also, on another tangent, Australian covers sometimes have a "20" scribbled in crayon or pencil, bulk rates were discounted going to the same area, a bundle of 20 covers were bound by an rubber band, postage discounted, and 20 scribbled on the top cover.