Hi Greg,
Many thanks for posting the extra images, it is appreciated.
It would appear from these new images that it is possible that some sort of erasure, perhaps of an earlier inscription, has occurred. The file marks can be seen running over the top of the Lion Passant and perhaps this was the point where the missing London and date letter marks were lost.

Yes, it will be Sterling (925/1000) silver.
Jefferys and Jones were important silversmiths in their day, possessors of the Royal Warrant to King George III.
Trev.