J1 Vs. J15 Stamp Size?

Hello Ladies and Gents! I've recently picked my collection back up after 30 years or so and I'm going through what I thought were some used Postage Due duplicates. I came across what is a J1 or J15. It turns out that it's not a duplicate at all, which is nice. I compared the color to J19 and it might be a J1, but this isn't a color question. My question has to do with the size of the stamp. While the designs of the 2 stamps are the same size, the J1/J15 has much larger margins than the J19, resulting in the J1 being so much larger, they don't even fit in the same size mounts. What gives? I haven't read anything about the 2 series being different sizes.
P.S. I need to get my scanner to work!

*** Moved by Staff to a more appropriate forum. ***

Without seeing the stamp, I'd guess that it is a top or bottom margin copy. The bank note company's typically set the outer row of perforations wider.