Just prior to World War II Jaguar, known then as SS Cars, started using a numbering system beginning with the letter X for internal projects. X meaning experimental, XB for military chassis projects and XF to XK for engines. This numbering system has never been consistent and there appears to be many omissions and duplications.
Number Project XJ3 3.4-litre and 3.8-litre S-Type saloon cars (known to the Pressed Steel Company as 'Utah') XJ4 Designation of the project which led to what was publicly announced as the XJ6 XJ5 Modifications to the Mark Ten for air-conditioning XJ6 A V12 racing engine with four-overhead-camshafts XJ8 E-type 2+2 version XJ13 Jaguar sport-racing mid engined prototype XJ16 Jaguar 420 saloon XJ22 & XJ23 E-type Series Two XJ27 The Jaguar XJS XJ40 Second Generation Jaguar XJ6 (1986-1994) (as opposed to Series 2 version of First Generation) XJ41 Prototype Coupé replacement for the XJ-S XJ42 Prototype Drophead replacement for the XJ-S XJ50 Jaguar XJ12 Series Three XJ57 & XJ58 Jaguar XJ-S 3.6 litre XJ81 Second Generation Jaguar XJ12 (1993 - 1994)