That stamped number on that bass is bona-fide. I still think the number stamped on the end of the fingerboard of the Santana guitar is an after-the-fact detail. That's not to say I think it's illegitimate; I just think the original serial number was either obscured by a refinish, lost, and/or forgotten. I also think it's possible, given the scroll headstock, that the number wasn't stamped up there at all. And it very well may be a duplicate issued number too. There are few things about that guitar that cast doubt on it being a '73 completion... though I would absolutely buy that it did start then... no reason to think otherwise. That guitar was a 'seat-of-the-pants' build right there, and I can just imagine the back and forth conversations while it was in construction. I bet it took a while. Photographic evidence suggests he had it by '75, and that doesn't seem like an unbelievable timeframe.
LSB #73-28 by contrast, is a much more standard 1973 Alembic. (if there was such a thing yet) When I first saw it, I went a little bit crazy trying to pin down what number it was by process of elimination, just knowing most of the other 1973 numbers.
So there's my guess. Anyway, easy to see how a bass might have flown from the Chicken Barn sooner, with the number stamped in it that may have been destined for Carlos' very custom guitar here. It'll be interesting to see what, if anything, ever turns up.