Could be that the number is on the attached (screwed or welded) side on some of yours. I'd never noticed these before, but both of my Ampegs referenced above have them.
The SVT numbers are all near identical. Starting with the back panel number 732323-1, chassis number of the front 732325-1 and cord wrap number 732322-1. The B25B, I don't have a serial number handy, but it's a '70-'72, I think. Its cord wrap number is 732306-1.
Lots of reasons to see what others have and on what years, but the biggest one for me right now is that I'm checking into getting some wraps made for a '69/'70 SVT I have that's missing both wraps. If they all had similar numbers, I could have one put in, I think?