Today’s entry from A Concise Dictionary of Greek and Roman Antiquities is coactor.
Coactor. A name applied to collectors of various sorts, e.g. to the servants of the publicani or farmers of the taxes, who collected the revenues for them (Cic. Rab. Post. 11, s. 30); also to those who collected the money at a public auction (Cic. Cluent. 64, s. 180; Hor. Sat. i. 6, 86).
The OCD does not have an entry for coactor.