Greg is correct (I have edited my post above).
After a communication with Greg, I quickly realized that my recollection of the "dry fit" of the parts was not correct. I went out to again dry fit my seats, and then I looked in the Factory Assembly Manual (of course, I should have done this before I posted above).
The holes for the two brackets on the rear wall of the cab are there.
The eight holes on the top lip of base (seat riser) under the passengers seat are there (two holes on the front and two holes on the rear for the two bracket-and-channel to seat-riser pieces).
The four holes in the front side of the base are there for the driver's seat support bracket (two for each front support bracket to seat riser).
The 4 holes in the floor at the rear of the driver's seat front support brackets are not there (two bolts through the floor at the rear of each of the two front seat supports). As described in the Factory Assembly Manual, there are 4 very obvious dimples in the floor - drill four 3/8" holes.
The holes for the cover panel are not there (sheet metal cover for the exposed top of the floor-base to the left of the driver). After installing the seat, locate the cover panel and drill the holes in the top lip of the floor base. I think that my cover has an extra "previous user supplied?" hole in it. My guess is that there is a hole near the front left edge, a hole about 1/3 back on the front left edge, and two holes at the rear.
If you have the 1947-1955 Factory Assembly Manual, look at SECT. 0, SHEET 4.01 and SECT. 1, SHEETS 2.06 through 2.12
