My suggestion: it's the "mode change" code/character found in various IBM punchcard encodings, punch 11-8-7, represented by an upper-case delta.
Rendering this slightly differently from the regular Greek letter, would make sense, I guess. This may also explain the varying representations in the manuals: some would represent this like in common EBCDIC charts, as a delta, while others would refer to it as represented in the actual on-screen character set.
(There is also the rather common problem with special characters in manuals, where the font used for the chart doesn't comprehend the particular glyph, giving rise to alternative, often more abstract representations, as these symbols were drawn in later.)
Rendering this slightly differently from the regular Greek letter, would make sense, I guess. This may also explain the varying representations in the manuals: some would represent this like in common EBCDIC charts, as a delta, while others would refer to it as represented in the actual on-screen character set.
(There is also the rather common problem with special characters in manuals, where the font used for the chart doesn't comprehend the particular glyph, giving rise to alternative, often more abstract representations, as these symbols were drawn in later.)