afaict That was a “prototype”. Was it ever made available to anyone beyond the authors?

It was supposed to be released to run on Z80 machines. Apparently it was killed when Smalltalk-80 was starting to be released (if Ted Nelson is to be believed):

https://archive.org/details/RosettaBrochure/RosettaBrochure/...

They wrote: Rosetta Smalltalk was shown at the Exidy booth at NCC'81 and beta-tested at about 40 sites across the country.

But it was unfortunatelly discarded as soon as Xerox published Smalltalk-80.

Fair enough, I don't wish to diminish their work, or lionize Smalltalk-72 over Smalltalk-80.