I tried all that you suggested.
The original part number for the Grate Ring was: 8-6 It was replaced by sectional grate ring castings. The right section part number is #42267 and the left section part number is #42268.
I will keep the 523, unused of course, in the garage until I can find the grate ring sectionals. If I can't find them, I'll try and sell the usable parts to others that need them. The grate and grate slide are used, but in good shape as they were replaced. The shaker handle is almost new and most other parts are certainly usable.
Maybe I'll be lucky!