Update! Adding to the record in case future members using similar Scooter99 sidecar mounts may find this info helpful...
I (finally) trudged through the snow to the barn and explored mounting options for this under-floor bracket with a few different old
scooter model (project or abandoned) frames that I have kicking around.
This particular bracket presents a problem for VBB or GL era frames due to the middle box section/floor support getting in the way of the bracket actually touching the floor. The bracket can't sit flush with the floor's underside because of the middle floor support. And, a VBB's centerstand holes on either side of the tunnel don't line up with this bracket's thru-floor bolts. New drilling req'd for that.
Not the case with P Series frames/floors. They would be fine with my bracket -- it formerly was on someone else's 2005 PX150. I have a very roached out, rusted '79 P125x frame. These Ps didn't have a center floor support, and their front and rear supports nestle nicely with this Scooter99 bracket.
PITA, but I would need to modify any VBB era bike for the sidecar. I'd have to 1) remove the center floor support so the bracket can sit squarely against the underside of the scooter, and 2) drill new centerstand type holes. Could I get away with removing the middle floor support and sealing the floor up, then attaching, I wonder?
Or, I'd have to pad out the bracket itself somehow so it supports the VBB floor properly in the front and rear, and also drill new holes and
use longer bolts. I'll give that some thought. Nah, it might throw off the bracket's swingarm bolt mounting too.
So I assume Scooter99 made/probably still makes a bit different attachment brackets that fit older 60s, 70s Vespa models' floors correctly.
Anyone in the states want to trade their VBB one for a PX one?
BTW, when the Scooter99 floor bracket is added to the frame, the original Vespa motor swingarm bolt is too short by about a 1/2" at 9-1/2" long (241mm). A bolt length of 10" (254mm) or a tad longer should suffice.
Scooter99 bracket on project VBB frame
... but it's offset with the middle floor support and centerstand holes too
Stock Piaggio swingarm bolt a lil' short with bracket on...
This random one's better but not strong enough for peace of mind. I would replace with beefier one...
Supports the floor properly and the centerstand holes line up.