ART, TOM - Been my experience that the "GUM DIPPER" Firestone tires last longer without weather checking & cracking. Since all my GY tires on my old CC's are cracked & checked, but the ONE Firestone 13.6X38 23 deg. on the '51 M that was installed about 1964 still looks almost brand new.
The fact those tires are FS as opposed to GY is a Good Thing.
MARLIN - I'd have to see one of those hydro plates in person to tell for sure but they do have a pretty nice finish, ground for sure, not milled. I used to have a calibrating plate for setting profilometers. It was divided for milled & ground surfaces.
If any type of repair would be done to Norm's plate, braze or weld, the WHOLE surface of the plate should be ground to flat and to finish.
The finish almost looks BLANCHARD ground with the grind pattern having arcs due to the round grinding head of the Blanchard, but it must have been a REALLY big one.
Years ago I used to know where EVERY Blanchard grinder was in the Quad-Cities. Most of them were at the machine shop the company I worked for rented office & warehouse space from. Couple others too. They're great for removing metal fast, getting things relatively flat and smooth, and with some rigging can get things parallel and square.