Here is a tip that has worked very well for me. As an astronomer, I use coloured filters to enhance detail on the planets, and I discovered that the standard Celestron 1.25-inch eyepiece RED filter drasticaly improves contrast with pickelhaube inkstamps, making them quite easy to read. It darkens the ink and makes it stand out against the leather, even with darkened leather, or at least it does for me. I was quite unable to read one stamp with a magnifying glass and a flashlight, but using the red filter, with the glass and light, I could see it perfectly. Now plain red coloured glass or gel might do exactly the same, I don't know, but it's worth a try, or even perhaps red light, like a bike back light. Anyway I thought I'd put it out there for what it's worth.