Joe, as I understand it, the belt drive SBs have a handlebar lever that operates through a Bowden cable to engage the clutch that is built into the drum drive pulley. The 430s at least, also have a "Park-Drive" lever that tensions the drive belt. The lever sticks out from the top of the drive cover. So for the 430, you start the engine in Park, shift to Drive when you want to start the reel rotating, then squeeze the handlebar lever when you want forward drive of the drum. Of course for close manoeuvreing and backing up, you just push it with the handlebars without engaging the drum clutch.

If some of the mowers don't have a "Park-Drive" lever, it seems like they must have a lever operated clutch in the engine output shaft, like the chain drive SBs, or there would be no way to run the engine without running the reel. I don't know about you, but I'm not going to tune up a mower with the reel running.