The C2242 cylinder head was designed by Harry Westlake of Westlake & Co, Ltd. and was originally cast in the UK by William Mills Ltd. of Wednesbury and West Yorkshire Foundries Ltd. of Leeds. It is made of RR.50 aluminum alloy which was chosen over other materials to save weight (50 lb. vs. 120 lb.)
During the first two years of production, the XK120 was fitted mainly with cylinder heads forged by William Mills. The West Yorkshire produced heads found their home primarily in the Mk. VII. Determining the origin of the two foundries” cylinder heads involves looking for differences in the casting around the middle spark plug drillings. William Mills heads were in two elliptical troughs while the West Yorkshire heads featured four separate circular troughs. In the spring of 1953 the two casting forms were unified with the 4 separated trough design prevailing. At this point the casting marks (WM and WYF) were the only way to differentiate between the two foundries, though some early heads did not carry any marks.
(Reference: Jaguar XK120 The Anatomy of a Cult Object by Urs Schmid)