is it a giant leece neville alternator? these tend to be most common that i've seen on buses and big trucks. they are massive compared to a regular automotive alternator.
they all tend to look the same to me, but come in different amperage ratings. some as low as 105 amps, and some as high as 300 amps.
mine was a lower amperage alternaotr. when i had it rebuilt it was upgraded to a 190 amp unit. That's a lot of amps!