The transmission and rear bearings in these tractors are standard metric bearings. If you have one out you can measure it in millimeters and then look it up based on the dimensions.
Ball bearings follow a conventional sizing system that was laid down over a century ago. You can use any brand but will have best luck with a name brand such as SKF, Timken, MRC (owned by SKF), Bower/NTN, Koyo or Nachi.
Decades ago IH made their own bearings but these days they buy them and slap a label on the box. The last time I bought a transmission bearing from IH it was made by Bower/BCA.
Suffixes and prefixes around the bearing number tell if it has shields, seals, snap rings, etc and tend to be brand specific. You can look up manufacturer's sites to find out what their nomenclature is.
The two basic differences that will affect you are ball count. Standard conrad bearings have fewer balls and no filling slots and "Max" type have filling slot grooves cut into the outer race and more balls added. It has to do with the way they are assembled at manufacturing time.
For axle bearings you'll want the type with maximum balls which have higher load capacity.
I don't remember offhand what the size is for a 300 Axle but I want to say it's a 212 or 213. An M, for example, takes a 214w (The "W" is Timken for Max Type, for MRC it would be 214M) and I know that the 300 is a little bit smaller axle. If you don't have it apart yet, note that the surface the bearing rides on is a different diameter, being metric, than the part of the axle that sticks out of the housing ( 2 1/2" on a 300 IIRC).