An Advance Ball is a small glass marble that rides next to a cutting head and allows the head and stylus to remain at a constant depth.  It is used to prevent over/undercutting on warped recording blanks.

Advance Balls for Presto Cutting heads came attached to the front shell of a 1-C or 1-D head.  The shells and advance balls were available seperately and could be installed on standard heads by replacing just the cover.