If you have several older stamps you may want to by a Scott US book, if you look around you can finder older ones for free or a lot less money than the newer ones. In this case there are 6 different versions of this stamp. #18, #20,#21,#22, #23 and #24. I don't think I missed any. The perforation should be 15 1/2.
Maybe some one else would take the time to explain the different versions. Or you can go to this link in our forum for some of the differences in this stamp.
https://goscf.com/t/10112