Hi Jeff,
Yes, it's a Post Office special issue air mail letter sheet from 1953.
There are at least four varieties!

If the flaps are cut straight diagonally across the corners then it's APS2a in Collect British Postal Stationery.
If the flaps are cut on a concave curve across the corners then it's APS2b in Collect British Postal Stationery.
The straight corners are more common than the curved corners.
Higgins & Gage describe both of these but list two varieties of each depending on the relative alignment of the words LETTER and AVION at the top left.
Your one is either H&G F12 if the flaps are cut straight across the corners or H&G F12a if the flap corners are curved.
So what shape are your corners?

All the best,