Do you have any other pictures without so much dirt on the reverse? It is covering a lot of the lettering. I am not completely certain, but it looks like 13-J to me. If that is correct it is a better variety, R6.
Your 1786 Maris 13-J had been entered in my database back in February, somehow I never replied on your Today's Finds post, just do not remember the circumstances. It is still the only one of that variety in the survey, so a strong Rarity 6. I have no idea on value due to its corroded condition......