The aftermarket Ranger style seat I bought from Baxter many moons ago to use on my '67 P11 has a fiberglass pan with a long tab on it at the front. As mentioned by Ron, the tab goes under the fuel tank. The short seats for the P11 were at the time not available, so I opted for a Ranger seat.
I used a P11A rear loop on my P11, but the brazed joints on the mudguard strap eventually cracked from the weight on them. My P11 frame is missing the heavier loop that would normally take the rider weight just before the shock mounts. I should have a loop welded on where it goes, but don't want to deal with a welder or tear the bike apart. So...
I made a set of brackets out of 1/8" aluminum flat plate that are riveted to the bottom of the pan and slotted to drop down over the shock mount bolts inside of the frame. Those brackets take the rider weight. I don't use the flimsy mudguard strap for anything but holding up the rear fender, or mudguard.
Below is what the bottom of my seat pan looks like. It aint pretty, but it works.
I should cut off those two studs since they aren't doing anything useful anymore.
