Last weekend's sales yielded this Starrett 429 twin-beam marking gauge at an estate sale:
As best I can tell, it came originally with one long and one short leg. My attempt at a picture of the face of the gauge was out of focus, so you'll have to believe me that it uses wheels, not pins, for marking. In common with the Stanley 98 gauge, pictured below, the face of the gauge fence has a recess allowing one wheel to sink below the face of the fence, so you can use it as a marking gauge (one line) or a mortise gauge (two lines). I've never quite understood the attraction of twin-beam mortise gauges: they look like they'd be hard to set for width, and the lines will start and end at different places. But some mechanics and woodworkers must have liked them, because lots of manufacturers made them.
As to my Crocodile Dundee reference: this is a hefty gauge. The Stanley 72 twin-beam gauge, in beech, pictured below, weighs under five ounces; the Stanley 98 twin-beam gauge, in cast iron and steel, weighs eight ounces.
This Starrett, on which the fence seems to be made from steel, shows up at 12 ounces - 3/4 of a pound.
I assume the octagonal shape of the fence face was to keep it from rolling on the bench.
