My local farmer's CO-OP has those bearings for $1.78 each the last I checked. The same quality you get in an assembly.
When I assemble my own I know where I stand on axial thrust that can be damaging during high speed Asian assembly methods.
To understand all this mumbo jumbo you have to understand what takes place with that internal spacer, its function and why it is just a few thousandts longer that the distance between the 2 outer race shoulder seats. There is a simple way to be sure there is no, none ( zero ) axial thrust at assembly time and also when the blade nut is torqued and I take that extra 3 minute step. Ball bearings of this caliber don't survive a natural life time with axial thrust.
All mower shaft designs don't have this spacer ( some craftsman ) but I've never seen a Murray without the spacer.
0ーF