It's a makers mark. Hallmarks are info on metal purity, production date and origin of an item that's made of gold or silver. As for the swastika symbol my thoughts are it is the Navajo whirling log symbol. I'd looked towards Navajo pottery makers for the information on what that piece was. I've never seen any native pottery with an under glaze mark like that. My guess is there was a pottery production maker, producing pottery for the tourist industry in the south west before WWII. Many of the the Navajo had also discontinued the use of the whirling log symbol after the rise of the Nazi, due to their unpopularity. It may not even be Native American, but a pottery company using Native American symbols on their product just to capitalize on the popularity of things like that in the 1930's.
I think is pre Nazi era?
