I had one and I do think I remember the number being stamped on the bottom-bracket. I know for a fact that I wrote the frame serial number down, because I still have that documented. Unfortunately can't locate any photos of the underneath of the bottom-bracket.
BTW: I sold mine last year. Great frame, but decided to move to a World Troller instead.
Here's the document image. I did what the instruction said and stored the number:
Here my frame in new condition:
