With my 1954 XK 120 came a 1952 JL gearbox that had a different Top Cover, not identical to the “average JL” gearbox that I knew. It turns out that such a Top Cover is correct for the initial series JL gearbox as used on the XK 120. May be the information below is also of interest for other readers.
Position of the Reverse Lamp Switch on early JL gearboxes of the Jaguar XK 120.
The first 4347 JL gearboxes still used the “old” Top Cover Jaguar C.1927 continued from the JH (and SH) series gearbox. These gearboxes are numbered JL8806 to JL13153. The difference with the later ones is the position of the Reverse Lamp Switch (C.1083 and Lucas 31077) which is located at the rear end of the Top Cover (gearbox lever side); see picture below.
From gearbox JL13154 this switch moved to the front (see pic below).
The change of position necessitated a new Striking Rod for the reverse gear (that operates the switch). Old rod was Jaguar C.878 and was replaced by C.7358.
Note that the JL series gearbox was used on both the XK120 and the Mk VII, so the exact number of XK 120s with the old JL Top Cover is not known. This gearbox was used approximately from January till October 1952. Taking the XK120 production figures per month (Philip Porter in Original Jaguar XK), we arrive at a total of about 2140 XK 120s (both OTS and FHC) that had this type of gearbox. This implies that roughly 2200 of these gearboxes were used for the Mk VII. The JL gearbox for the Mk VII has a different Rear End Cover but is otherwise identical.
Introduction of the “stop pin” on the 2nd speed synchronising sleeve
Around the same time, Jaguar introduced a “stop pin” to prevent that the 2nd speed synchronizing sleeve could move too far back, causing the 1st gear to become stuck. This “stop pin” was introduced on gearbox JL13834 which dates probably November 1952.
See also the excellent article on this subject by Rob Reilly