Richard Morton is quite an interesting maker as he registered several marks:

September 16th 1773 – “RM”
September 30th 1773 – “RM & Co.” and “RM”
July 13th 1780 – “RM”
February 12th 1781 – “RM&Co.” and “RM”

Click on a picture for more details

1778

1781

1792