Just checked a War File DVD on Churchill tanks and this was mentioned. It seems they never made it to war. The Black Prince was the name assigned to an experimental development of the Churchill infantry tank with a larger, wider hull and a QF 17-pounder (76mm) gun. It was named after a 14th century figure, Edward, Prince of Wales who fought at Crecy. The slow speed of the Black Prince and ready availability of M4 Shermans which could be upgraded as "Fireflies" with the 17 pounder gun limited this tank to experimental status. Six prototypes were completed.