Comment by masswerk

My guess: it's the "mode change" code found in various IBM punchcard encodings, punch 11-8-7, canonically represented by an upper-case delta.

It would make sense to render this somewhat differently from the regular Greek character. This may also explain why it's rendered differently in various manuals: once, as commonly represented in EBCDIC charts, as a delta, once, as it's actually represented in the on-screen character set.