UK - Souvenir Sheets Or Postal Stationery?

Hello All,
Below are four examples of what I first thought are some sort of commemorative British postal card, but the small text on the cachet reads "This Souvenir Sheet has Postal Validity". They are printed like a postal card, but with thinner paper and a serial number stamped on the back of the first three. They date to the mid-1970's. What are they really? Thanks, Will

Actually both !

They are Postally valid sheets made by cutting down Postal Stationery envelopes and stamped-to-Order.

The bottom example was originally issued to celebrate the 100th Birth Centenary of Winston Churchill, by the organizers of the 1974 Southahmpton Stamp Fair.
I can just about see the laid line Watermark on the paper and there should be One or more Phosphor bands over the stamp Impression and have no gum.
Again, regarding the bottom example,the overprint was added in 1976 to use up residual stocks for presentation purposes, probably by someone connected with the 1974 fair.
Most of these sheets/stationery envelopes, were issued for Philatelic Exhibitions.

Londonbus1

Oh yes, only 500 of the 4½p Machin were prepared making it quite scarce.
With the overprint, even more so.

Now I'll shut up !

Londonbus1...can talk about Exhibition stuff all night

Souvenir stationary . . . who knew?

Thanks Londonbus! Just when you think you've seen it all, always something new.