Hallmarks on golden ring

by Marjolein
(the Netherlands)

I have a gold ring with hallmarks on two sides (see picture). On one side there is a black square and 750 (which I presume to mean 18 kt gold. On the other side there is a square with the letters 'ram', again 750, a not-closed diamond and the letter y (a year letter?).
What does all this mean? I am very curious.
Thanks!