Ken,
On 9/27/2017 11:10 AM, Ken Shirriff via Unicode wrote:
The IBM type catalog might be of interest. It describes in great detail the character sets of the IBM typewriters and line printers and the custom characters that can be ordered for printer chains and Selectric type balls. Link: http://bitsavers.org/pdf/ibm/serviceForConsultants/Service_For_Consultants_198312_Complete/15_Type_Catalog.pdf
That is a very interesting source, though from a much later era (1983). In particular, the "Special Character Nomenclature" (p. 11 of the pdf) provides a good list of what the IBM typographers at the time thought was the range of special symbols they were working within this overall collection.
Note the presence of the group mark, the record mark, and the segment mark. And in the realm of potential "tofu" indicators, there is the open box and the OCR blob, but nothing like the 1620 symbol(s) we've been talking about.
On another point, the "pillow" noted for the invalid character in the IBM 1620-2 (using the Selectric instead of the older IBM typewriter model) was almost certainly also not an actual punch on the Selectric type ball, but instead implemented by an overstrike of "[" and "]". See, e.g., the Pica 72 type style in the catalog noted above, which looks like some of the very earliest Selectric type. Its use could well have been occasioned by the fact that the slab serif typewriter font would have created a muddy blob if you tried to overstrike an "X" and and "I" for this output symbol.
--Ken
