Replacement bottom bracket compatible with BB-UN100
Any square taper bottom bracket with BSA threads (1.37inch by 24tpi), 68mm shell width, JIS taper and axle length roughly 123mm will work. 

Changing axle length from 122.5mm to 123mm makes 0.25mm difference to chainline, which is unnoticeable. Many stores list BB-UN100 bottom bracket as 122.5mm, so it is possible that there is actually no change at all. I could not find any Shimano documentation for the letter codes, seems that Bartnett's explanation does not match reality.