Mk I used barrels from BLC guns Mk I/IV; Mk II & III used barrels from BL Mk IV or VI; Mk IV used BLC Mk I/VI adapted for Mk IV carriage; Mk V used QF Mk II barrels converted to BL (total of 63). Details from Clarke 2005, page 34, and Tony DiGiulian's website Britain 6"/40 (15.2 cm) QF Marks I, II and III
