This is the Girling slave cylinder used on the genuine GT40 serial numbered cars as well as many racing cars with small bellhousings. There are three bosses cast and machined into the bellhousing to perfectly align the cylinder with the clutch release lever. Three points define a plane. Two bosses are drilled and tapped for fasteners, and one boss supports the cylinder in the plane. The bellhousing and slave cylinder are designed, engineered, and manufactured to operate together and align all of the clutch components.