HI!  I have a set of sprocket covers off of a 35, that will fit the 35, 40, and R5.   I would trade them for a set, or just the center castings, part number 1A5525 for a Twenty Eight or Twenty Five.  Picture of the 35 covers and a picture of a Twenty Eight cover for reference.
Contact by PM or here.
Thanks
Ken   
 

I am thinking that should fit a RD6 too. Only a guess since I have no parts books. But have seen many CAT 35 and 40 parts that do fit a RD6.

I bet there is some weight in them, look at the thickness of the sheet steel. They would be a interesting part to duplicate, looks like the part you step on is cast steel, riveted to sheet steel, there's probably a few folks Stateside and here in Australia who would buy a set of them to finish a restoration. If a pattern was made for the cast section, anything is possible.

Hope you find the parts you need Ken.
Mike

Forgot about the RD6.   Yes they will
Thanks
Ken

Yes Mike maybe someone in Aust. May need some , I had some Cast Years ago . nobody wanted them , eventually i sent them to NZ. to a very grateful collector. just remember there are not that Many Tractors out here so market is some what very Limited . LJ

Where are they at.
doug