Two-tailed cat stamp - what is it?

Dan Watson writes to say his grandmother was interested in stamps and found these "Two-tailed cat stamps" among her collection. She had marked them as "rare."

Dan says he can't find any information and was wondering any Round-Up readers might shed some light on these.

Originally, I thought it was a color shift of some sort but the black is not shifted elsewhere as you can see in the close-up. Scott does not list it as a variety.

Anyone have any ideas as to what caused this and what it might be worth?

If you do, write me at donschilling@worldnet.att.net