OK, now I see what's going on... sort of. Apparently, I've broken off the second plastic tab there in the PTO clutch lever slot! (see pic 2) And yes, moving the lever slowly forward, there does seem to be an internal detent there as a mid-position for the lever, just at the point where that tab is missing.
Guess I do remember it being there, but certainly never missed it -- I just slide the lever right on past it to be either fully forward or fully rearward. I always thought it was just a second safety to ensure you had to be deliberate about engaging the PTOs.
Went out to the tractor: the front PTO shaft (snowblower mounted) will turn easily by hand no matter where the clutch lever is positioned, if the "2000 RPM" lever is not engaged. As soon as the "2000 RPM" lever is engaged, the shaft cannot be turned no matter where the clutch lever is positioned. Didn't fiddle with the rear PTO shaft because its plastic cover is on. (This was with the tractor not running.)
Went into the manual, and it doesn't mention a "PTO Brake" anywhere, just says to move the PTO clutch lever forward to engage and rearward to disengage.
So-- I can't tell if there's a PTO Brake or not, but there was a second plastic tab there, near the center of the clutch lever slot, that's been broken off.
As far as my tractor, yes, it probably is a late 2007 -- It's just about to have its "birthday" as I got it in Feb of 2008, and I do remember that the "new, improved" 2400 series came very soon after I got my 2310.

