Your thesis, as expressly stated and reiterated, was that Smalltalk didn't include low lines "because it simply _couldn't_. The fact that it was possible to have used Smalltalk on machines that "couldn't" in 1980 does not prove your point any more than the fact that it is equally possible to use those machines today.
I *do* so wish that people would take the rather minor trouble to read carefully and think about what they read.
My point about the 1980s VMS printers *of course* doesn't prove anything about Smalltalk-80, nor was it intended to. *READ IN CONTEXT* it quite obviously was intended only to rebut the claim that ASCII was everywhere in use and everywhere the same.
The fact remains that the character set actually used by Smalltalk in the coloured books is suspiciously close to ASCII-63. The only characters *not* in ASCII-63 that it uses are the lower case letters, and it was obvious when ASCII-63 came out that not only were the lower case letters going to be added later but that they were going to be added where they now are. (The FIELDATA precedent for aligning the lower and upper case blocks was clear, and the reasons for doing it still applied.)
So what what _was_ on the Alto keyboard?
Long pause while I find an Alto BCPL manual. Annoyingly, The "BCPL Reference Manual" by James E. Curry and PARC staff, compiled on September 14, 1979 (note date!) does not list the supported character set, and does not spell out the lexical structure of the language. However, comparison with other BCPLs reveals: [ instead of { or $( ] instead of } or $) % instead of | = instead of := uparrow used in a non-standard feature baStudlyCaps identifiers (not normal BCPL at all) no SEPARATED_WORDS (which _was_ normal BCPL practice) With the exception of = for :=, the differences again point to ASCII-63, which _did_ have uparrow, but _didn't_ have { } or even | (in fact, the ACK control character was slap bang on top of where | is now). Even more interesting: I can find no characters used in Alto BCPL (other than the lower case letters) that were _not_ in ASCII-63. Now ISO apparently announced in October 1963, just four months after ASCII-63 was released, that the lower case letters were going to go where they are now.
It's not without significance that Mesa, the other systems programming language for the Alto, also used left arrow for assignment. I haven't been able to get my hands on a Mesa language reference manual yet, sadly.
However, we do have evidence that the Alto character set was (possibly an extension of) (October 1963) ASCII-63.
What we think of as ASCII is ASCII-67, which came out in 1967. The adoption of a new standard is never instantaneous, so even in the early 70s, you could expect to find ASCII-63 around. Like on Teletypes. Like, even in 1979, on Altos.