The description I looked up said it requires Exmark #135-7013.
That's the plate atop the fuel tank that has the special slots. Thanks for that PN, it helped a lot.
PART II
After some serous thought and new mower shopping, I decided that the
Seat Kit, expensive as it was, was a better choice than buying another (new or used) mower. Since I have the 44" Radius, the change required the
126-9868 Kit, plus the afore mentioned
135-7013 Fuel Tank Plate.
Installation is easy and straight forward. First you remove the existing
Fuel Tank Plate which is held in place by 2 bolts.
Then install the 135-7013 Plate using the same 2 bolts.
Then install the new Seat Plate from the 126-9868 Kit. Once that's secured you can fasten the existing seat to the new Seat Plate. This is made easy because the seat now hinges forward to expose all the fasteners.
The new seat plate hinges at the front via existing holes in the L&R control lever columns. The gold rod is the hinge pin.
Lastly, a spring-loaded safety latch is fitted to the frame on the battery side to keep the seat from hinging forward during a roll over.
Total installation time was about 45 minutes and only required 3/8 and 1/2" sockets. There were no special knowledge or tool requirements. The Kit raises the seat about 1" over the previous position. All the seat's front-back slide adjustment is retained. The ride is much, much better, but not superb. Still, it's far better than no suspension at all.