Ciao Krisztian
I saw your post and did some research, unfortunately to no avail. The Milan's marks seem genuine, but in the books I have, no silversmith with that mark is listed.
Gianguido Sambonet, in his book "Gli argenti milanese" records the existence of a workshop dedicated to the axe, run by a certain
Gaetano Premoli, watchmaker.

One might assume that the mark on your stoup refers to that shop run by an unknown G.S. after Premoli's death in 1854.
These are just guesses.
Regards
Amena