It's not a flat plate. It's about three inches long and 3/4 inch thick and machined as an open end wrench in four different sizes. On the end is a square hole for a BSA axle end.
I heared a story from an old six-day rider once about a Dutch rider who took a bad fall and had the letters BSA embossed on his rear end from the tool in his pocket.
Leonard Bulger A^2, MI
Chuck asked: Is this a flat steel plate with a bunch of different hex-holes in it?
The track "jamming tool" I'm familiar with is a six-inch wood dowel that goes in the back pocket of the shorts for your team mate to grab when he gives you a "madison throw" into the race.
Chuck Schmidt
South Pasadena, Southern California