Damon Atchison writes: "[The] material is red and flat....and is
somewhat like a felt.  Can anyone identify it and tell me where I
could find the exact same material to replace it?"

It _is_ felt.  Reed organ manufacturers often used this economical
alternative to leather.  A pump organ (non-player) can get by with a
little leakage.  Felt compresses to the thickness of card, when used
for gasketing.  I do not recommend it except for originality or a
situation liable to warpage: felt gaskets, while forgiving, can't seal
as tightly as cork or leather.  Wool felt is soul-food for moths.

Nameboard felt coming in wide strips/sheets works well.  Cut it wide
enough to cover the whole joint in single-width strips.

Robert Linnstaedt
The Organ Shoppe