Factory errors are just that. Since they were done by hand back then, errors were made. Quality control usually caught them, but once in a while something slipped by. Does that make it rare? Perhaps, but it's value is only determined (like anything) by someone's willingness to pay for it. As far as I'm concerned, errors like this are interesting, but that's all. Your Pacific is probably one of those pieces that slipped by the quality control people. It's also possible that it was repainted so long ago, that the paint has aged accordingly, or as has been pointed out, is a replacement shell (which in itself makes it interesting). But it probably is a true error as you described. Some folks collect them, and I understand that. It makes life interesting. I came across a 336 Northern that has no lettering on left side of the tender. I keep it around because it runs nicely and makes a good conversation piece.