DTD224 was cancelled as "Obsolete"; up to mid-1940 80/87 was the standard (then 100/130) so "77" may have been a pre-war substitute.
There were higher grades like 150 in some Eighth AF P-51s and 160 tried in a Spit VIII for "Basta" trials.
This is a 1944 DTD listing, Australian printing but UK M.A.P. material specs. I'm only really interested in those relating to "aircraft finishes".
http://silverbiplanes.com/SPECIFICAT..._BY_NUMBER.PDF
Regards
Nick