Ampeg V4.com

as with most of ampegs 60s amps,The B15 was such avery  fluid design,  you can open up 2 b15NCs and find at least a few minor differences between them,  and the schematic. 
there is an interview with jess oliver here:
http://www.keithrosier.com/jess.asp

the interview is excellent.  Jess Oliver is brilliant.

anyway... he says this
 

"Model A-15, the A stood for Accordion. It was the same as the B-15 but accordion players liked to be assured it was designed for accordion. The B-15, B was for bass. The B-15N, the N stood for new engineering changes. The B-12X and B-18X, the X stood for extra power or extra effects like reverb or vibrato. As for the B15ND, I think that was the same circuitry only made with a printed circuit instead of an eyelet board. I'd have to dig up the schematics to confirm that."

the ampeg story behind the sound book is quite vague on the B15ND,  That book is great... but they should make a 2nd edition with all the errors corrected and some added information.

"Model A-15, the A stood for Accordion. It was the same as the B-15 but accordion players liked to be assured it was designed for accordion. The B-15, B was for bass. The B-15N, the N stood for new engineering changes. The B-12X and B-18X, the X stood for extra power or extra effects like reverb or vibrato. As for the B15ND, I think that was the same circuitry only made with a printed circuit instead of an eyelet board. I'd have to dig up the schematics to confirm that."

I think the second letter could be considered more like a revision number.   
The B15N could be considered the rev A,  then B15NB would be Rev b and so on.   
the big difference between a and b was the switch from tube to solid state rectifier.. (which they changed back to tube in the B15NC)