M-number vs R-number Differences

I was doing a search on TractorHouse for some old IH parts and came across something odd. I was searching for 583 588 R91, which was supposed to
be a shaft support. I also found a 583 588 M91 part. The R91 part is $27, the M91 part is $122. What is the difference between M-number and R-
number parts? Is one an assembly and the other not? Or would these be two completely different parts?

"M" is out of the Ferguson-Massey Ferguson parts system while IH used "R" and "C" designations.

Suffix letter R and C is all I have ever seen on IH parts and I worked at Farmall and iH East Moline about 5 years so saw lots and lots of part numbers. IH did use the letter D as a suffix WAY back in the early days, say 1940 and before. Design revisions were made by adding a second letter like DA and DB.

Never saw a suffix M-1 on any IH part.

The Massey Ferguson suffix makes sense, I've never even seen a Massey part number.

I've ready many articles written by people claiming to be far far smarter than I claim to be about what makes a good part numbering system, and the iH system seems to be really really good. The seven digit part numbers, like 127-54-36 C91 (hyphens added by me, should be 1275436C91) were a HUGE step backwards. If parts were similar the left side part was an odd number, right side part was even, maybe it was the other way. But those seven digit numbers were TOO BIG, IH part numbers for decades, even a century had been 5 or 6 digit, but most people familiar with the parts only used the last three digits. With seven they would say the first three, then the last four as a single number then group of 3, or two groups of two, or sometimes the 4th digit alone, then last group of three. Eventually everybody carried a note pad and wrote the whole number down and used the sequence they were comfortable. But the IH drawing and revision control was really really good even from the very beginning.