Stamp on Gold & Diamond Ring

My engagement ring is a family 'heirloom'. It definately belonged to my grandmother and we believe it was her mother's before her (my great grandmother - Ma), although it's possible it belonged to Ma's aunt before that. We have had it valued and have been told it is an 18ct gold ring but the only stamp on it is a small 18CT, nothing more.

My great grandmother came over to Liverpool with her family from Ireland. As my grandmother was born in 1919, Ma and her family must have come over in the early 1900's, we believe Ma was a teenager when she arrived in Liverpool. Could you shed any light on why the ring doesn't have any other markings on it?

Many thanks

Lisa Taylor