#5 -- "57": This mark is found only in Enfield bullet bases. As stated above, I personally don't pay much attention to the stray "dots" that also show up on some specimens. These are fairly common finds. These were made by Eley Brothers in London. I believe the "57" mark is fairly standard and any odd variations are probably a result of being fired.
ADDITIONAL comments:
Yes, Mike, I remember seeing the "57" in the Dance & Park bullet. However those bullets were definitely cast, so the "57" in the cavity was not formed by the same means. The only reason I can imagine that they are there is if the mould maker was given a "57" base Enfield bullet to copy from and he did so to the last detail...including a "57" on the base plug.