A student has an Archer II that has a red arc painted on the glass of
the tach between 2150 and 2350 RPM. The POH makes no mention of a red
arc.

I'm thinking that either the tach came out of an Arrow (which has a
red arc in this range) or some A&P confused the Archer with an Arrow
and painted it on.

Does anyone know of any legitimate reason for an Archer to have a red
arc, other than max RPM?