Mokes do have that engine number. The F/A stands for low compression engine and was stamped on all English Moke 850 Engine Plates. Here's a well documented example at Sothebys With same Engine Prefix https://rmsothebys.com/en/auctions/cc19/online-only--drive-into-the-holidays/lots/r0007-1967-austin-mini-moke/818558

We have what i believe is a pretty early 850 in a pretty early MK1 MOKE. the number on the engine tag however doesnt match the Mini engine list here on this site. according to the decoder doodad https://www.minimania.com/engine___identification_data_updated the code should start with 8AC for a moke, but this one clearly says 8AN, which doesnt appear at all on the list, thought there were 8AM engines. Im also curious if the FA hand embossed into the plate means anything. Current the working theory is that this engine may have been rebuilt and a new plate made, with an accidentally transposed letter (from M to N) and the FA is the mark of the rebulilder?  any input is appreciated.