There were a few different iterations of double sided "Pocket Signal Disks" like this one--I've seen one issued by the Boy Scouts, one from WWI, and this one from WWII-- designed for practicing two arm Semaphore and International Morse Code. Tons of information is densely packed onto both sides, with interactive dials allow one to scroll through a range of "special flags" and "alphabet flags" which correspond to flag positions and codes. I really love the design and graphics, and especially love the phrase "Practice Insures Efficiency" arching across one end.  A cool thing, and if not to be used to actually practice one's signaling methods, perhaps inspiration for some sort of party game!

This one is in near mint condition, with original protective paper pocket into which it slips.  6" tall x 3 5/8" wide.