OHMS designation

[Lancastrian]

Can anyone please tell me when the OHMS endorsements on official mail began? I have some recollection of reading that it was with uniform penny postage in 1840? I attach scans of two covers, one from 1840 with manuscript OHMS to a Scottish procurator fiscal and one 1841, also Scottish, with printed On Her Majesty’s Service to a tax office.

[Winston W]

I can't tell you when the OHMS endorsements on official mail began, but here's one from February 1800, so way before your recollection that it might have been 1840.

[Lancastrian]

Many thanks for that Winston, as you say it clearly goes back further than I expected! Has anyone an earlier example?