In 1989 Patek Philippe launched their first “in-house” repeater movement--back then the Gerald Genta brand dominated the chiming wristwatch market. R27 had automatic winding, like the Genta and Blancpain repeaters already on the market, but was a bit thic...
AP added a system that blocks the crown when chiming, and an extra hook to carry the quarters rack (which skips the "phantom" quarters) to their MR movements.