[update: whoops, looks like indirect addressing may not be an option in the ARD instruction; maybe ARD into ST9:0, and then copy/move ST9:0 to ST9:[index]?]
If this were my system, I would connect to the scale with a utility program like RealTerm that would let me easily view the incoming data, and quickly experiment with different "serial framing" settings and data rates.
Bingo, this^ There are not a large number of settings so it should not take long.
Does Hyperterminal still exist (I used to do a lot of RS-232/RS-485 protocol work with TV/broadcast industry hardware)?
the next thing would be to write a very simple ladder program that dumps 17-character strings (per section 9.4 of the manual; see below; and that may not be the right manual, of course) into a dozen or so ST9 strings, with ST9 acting like a circular buffer, (ARD(ST9:[index],17), then index:=index+1; if index==12 then index=0.). Once you have recognizable data in ST9, then the fun begins.
Btw, Ken, along with the bad character count in the 1400; is it possible the high bit being set in ST9 characters indicates errors as well? This is ASCII, and "true" ASCII is 7-bit*, so that high bit would always be 0 and essentially ignored in good data values anyway, and so available to be used for summat else i.e. error indicator.
* although we treat LATIN1/ISO-8899-1 encoding as "8-bit ASCII"