I managed to get in touch with Eric of Hand Tool Rescue. He had this to say:
I think what you have here is a Stanley No 8 (Type 7) body and lever cap, with a Type 12 blade, and an early Union plane frog (see attached). It was probably added on not long after production as Type 7s were made from 1893-1899 and Union did not start making planes until 1900. What you have is definitely quite old and just the frog itself is rare.