patent 183608
if you read the complete writeup on the patent, in the 2nd column it says the top piece mat be moved so as to extend the rule for taking intermediate lengths, such as for doors, windows, stairs, etc.
the parts can be set to form any bevel desired. an d may be used also as a yard-measure, lumber gauge, lumberman's measure and other uses.
did yours come with two wood rules ? it looks like two, one on top of the other, and you would loosen
the screw and slide one out to take extra measures. like the mentioned of using it in a door way.
I hope this may help, Frank