Made in our own machine shop to precise tolerances, they accept set screws, machine screws, hanger bolts or threaded rod with standard 8-32 or 1/4-20 threads (you supply). A bit of adhesive on the threads will let you anchor the thumbscrew to a fastener shaft.
The 8-32 thumbscrew is about 3/4" x 3/4" wide, with 1/4" of threaded depth. The 1/4-20 thumbscrew is about 1" x 1" wide, with 3/8" of threaded depth.
Made in Canada.