I just found this and two other old folding rulers and am completely in love with them; I keep carrying them around as they are just a complete delight to hold in the hand, and how much better to measure little things with one of these than with a clunky measuring tape! I am pretty certain this one was made by Stanley and, though unmarked, I believe it is model "No. 32", and am certain it is the youngest of the bunch, 1920s-30s I think. The numbers and markings on both side of the ruler are super clear, and this one has a 3 inch caliper that pulls out at one end, marked with sixteenths of an inch. Super functional and a total treat to use, It unfolds to 12 inches via two unfoldings, with one of the original pins in tact for holding the long sides together. 

12 1/4" x 1/2" x 1/8" when completely unfolded; 3 3/8" x 1" x 3/8" when completely folded. The wood is great condition; the brass on the interior, which I have not touched, shows some aging but looks like it would likely polish up some if desired (tho I think it looks pretty great just as it is.)