Currently listed on eBay for $500, the 1936 Imperial Roto-Gage Stamp Perforation Tool is similar to the one that President Franklin D. Roosevelt used to measure perforations, except his didn't have the attached magnifying glass.
According to the National Postal Museum's website, "This black plastic gauge, used by stamp collectors to measure the number of perforation holes in a two-centimeter length of stamp, was on FDR’s desk in the Oval Office from 1937 until 1945. A fifteen-year-old stamp collector from Illinois had given it to him."
This handy little item also had a watermark tray so, according to the instructions, "no time is lost searching for separate pieces of equipment you may have mislaid."
To see the eBay listing with additional close-up photos of the device, click here.