Tony,
Welcome to Treasure Net !!
Any numbers, words, letters, images or apparent means of attachment on the reverse?
Ooops, DCM, sorry, you beat me to the Q.
Don.....
The only Polish use of a swastika I could find dates back to the First Polish Republic, too long ago to be likely.
My thought is that it is of a Scandinavian design, pre-ww2. I'd seen one listing for a belt buckle of similar design said to be from the Scandinavian foreign volunteers helping the nazi cause in WW2, but couldn't access further information about it, and it was only a match in the style of swastika, not in the rest of the details anyway.
Thanks for the challenge. If there are other unknowns from your grandfather's past possessions that you'd like to ID, there are many TN-ers here who seem to be able to ID just about anything--so post them if you wish.
Don.......