Greek Influences in the VMS glyphs?

[Stephen Carlson]

2. Greek Abbreviation Symbol. The staircase symbol with a dot is a rare character in the VMS. It is a shape-mate for the Greek abbreviation for "atto". I've never seen this shape in a Latin-character manuscript and Latin abbreviations don't usually have this shape. I've only seen it in charts of Greek abbreviations, but there are a few abbreviations in Greek that are based on this general idea of a staircase+dot, so it might be a specific class of abbreviation.

What is "atto" (it's not a Greek word I'm familiar with or can find in a lexicon) and what's your source for this?