I was thinking that the cast side plates must be interchangable between some different models, the Gas 50 is 28.5" long, D40 is 27.5" long. As some of my 50 plates are broken, would one from an RD6 work?- I do know of a 3 cyl grader that is a likely donor. So far the two I have measured are both right at 5.5 " wide.
The diesel 35, 40 and RD6 are the same parts. Possibly the D50 would be same as your gas 50 but I don't have a gas parts book. The plates for a D50 are 2A675 and 2A676.