What dictates alternator amperage?

[Detour]

My 67' K20 327 has the factory style alternator that is stamped for 37A. I found a guy locally selling a NOS 55A for $10. I grabbed it and didn't look in the old greasy box until I got home. It is definitely a heavily used alternator that was remanned in the past. (Heavy aluminum color overspray in all the vents). I pulled it apart and it was full of crud and dust from the brushes.

Thing is everything looks identical between the old 37A and the recently purchased 55A. Housing, diode/electrical components in base, stator and rotor even appear to have the same # of windings. What components should I swap into my OE alternator housing to up the amps? Thanks