Colt uses an ‘&’ as an action mark to indicate that a revolver had been returned for factory repair or refinishing.

related:

• There is an image of an example ⇒ (scroll down to see marking) in Hallowell & Co.’s Illustrated Firearms Dictionary.

Footnotes / Sources:

Hallowell, Morris L. “Illustrated Firearms Dictionary.” Hallowell & Co., Inc., www.hallowellco.com/abbrevia.htm#A ⇒.