I'm fairly sure in this case:
-D, DA, DB, DC, DD, DE, etc. suffixes would indicate earliest to later versions of the same individual part.
-DX, DAX, DBX, DCX, etc. suffixes would indicate earliest to later versions of some form of an "assembly" or a partial assembly.
Usually, parts with newer suffixes should be able to replace the older ones, but older ones may not always be able to fit where a newer suffix is called for. Sometimes additional parts had to be changed to go foreward or backward with the part number suffixes to make something work.
There are exceptions to these "rules", and there are further "rules" that I don't think apply in this case.
I'm assuming that PN. 23603D, a dowel pin, comes with PN. 3281DA to form PN. 3281DAX. If so, and if the dowel pin wasn't used for the desired application, or you only wanted to order the saddle piece by itself, you ordered PN. 3821DA. What they changed to get from -D/-DX to get to -DA/-DAX in this case, I don't know.
IH had part number "explanation" books available to dealers. I think this was explained in an issue of "Red Power Magazine" once, too.
Neat piece, anyway anybody wants to explain it.
If I've totally screwed up the suffix explanation up or someone can take a better stab at it, please do. :)
This post was edited by AG in IN at 18:12:55 01/20/14 2 times.