The disc brake hub is an integral part of the the bicycle's rear hub. it has a machined surface, and studs or a bolt hole pattern to match the disc.
happy time driven sprockets are bolted onto the everyday bicycle hub through spaces between the spokes.
the main reason that STATON gearboxes last for thousands of miles is because the drive sprocket is not in motion for a large percentage of the bike ride. it seems logical that the longer the gears are in motion, the shorter the gearbox's lifespan, and vice versa.
the STATON gearbox's drive sprocket is not freewheeling. it's the driven sprocket on the STATON rear hub that has the freewheeling capability.
the STATON gearbox is 9 1/4" long, 5" high and 2 1/2" wide(3" at the clutch).
i have a spare STATON drive sprocket and a STATON driven sprocket. the sprocket attached to the rear hub is 5/8" thick with a 1 3/8" threaded hole. the drive sprocket is 1/2" thick with 5/8" keyed hole. just as i'd thought, the drive sprocket was heavily machined/resurfaced, so much that you can barely see the threads where the setscrews would've been.
because the setscrews' holes were machined away, the modified sprocket is retained by a snapring(circlip)on front of the shaft.
of course these sprockets are not interchangeable. you could probably find a 5/8" 18t freewheelsprocket, but to machine it down to 1/2" width and still retain the freewheel intact is a machinist's guess.