Vintage silver Pendant

Hello I was searching for presents for family & come across this pendant which was sparkly & I thought I would have a good look at it, just did a few home tests on the stone and think it’s real, had a look at the stones and they hopefully could be diamonds , I’ve noticed a mark that I’ve never seen before, definitely not British, looks very old, I took pendants off chains to clean them so no clues from that,
Hopefully someone has a idea what the mark means.