If your stamp is 10.5 x 12 you have a desirable stamp if mint. Not worth so much used but still more than a regular example. It would be Zagorsky 272 A. If it is perf 10.5 all around it was perforated privately and not worth much. In Michel a 10.5 x 12 would be 398 BX or 398 BY depending upon whether the watermark is horizontal or vertical.