Featured Roman soldier signet ring

That looks very much like a hallmark!

 

Certainly a mark, and the right one for its origin. Post-1934 Spanish maker's marks had a hexagonal cartouche.
It even looks like it was struck twice, which makes it difficult to recognize. There would be a number for the maker, and a letter for the city where it was assayed.
A post-1934 Spanish maker's mark fits in the timeframe and suspected origin.

Bego, could you check if there is a small star mark as well? It could be anywhere on the gold.

Very unlikely, we don't get much US made jewellery here in Europe. Besides, this style ring was also popular overhere, and we have a very long history of excellent jewellery making.
And then there is the mark, but that was posted after your reply.

 

That mark would have been hard to read even when the ring was new.

 

I have searched for a star mark, I found something else that could possibly be a mark, almost round though. Tried to get a picture, difficult (again) to get a good one. What do you think?

 

it does look like a punch mark from here.....and not on a place where a ring might usually get dented or dinged.....

 

I agree with komo, but unfortunately it is too small for me to make something of it.
I hope the confirmation that it is a post-1934 Spanish ring is enough information for you. It is not only a connection to your grandfather, but also to his native country.

 

Yes, thanks so much, this is very helpful!!