I don't think a backstamp can be put on the matting on the bottom of the full size piece. We put a special sticker on the bottom of the 100 Cedar Island LEs we sold for the 1st LI Lighthouse Challenge. I had the option of designing and making the stamp myself or have Harbour Lights do it. I did it myself which made it challenging to get it on the bottom of the first 50 presigned Bill Younger pieces. lol
We were able to get a backstamp on the Little Lights because they do not have the matting on the bottom of the pieces.

Bob