This is a frosted (and sometimes clear) Galle mark. It can be found on the bottom or side of the vase. Please be aware that we have seen similar marks on Galle reproductions. The exact mark we have shown is from an authentic vase though.
Please contact us if you have any Galle vases for sale. We also happily provide free value opinions and appraisals. Vases@GalleArtGlass.com