Jeff, Ian and all,
I have one vintage mower: a 1984 Toro model 20672 walk-behind rear bagger with blade-brake clutch (BBC).
What that feature is, when you start the engine the blade will not turn until you engage the bail bar; let go of the bail and the blade will stop, but the engine will continue to run - useful if you want to move things out of the way of the mowing path or to empty the bag between mowing sessions.
And for Jeff: do you wish to find the same Clarke-badged, red-painted model 153 that Ian has? His is a 1976 model.
~Ben