By 1966, Jaguar realized that there was still a market gap between the 3.8S and the MK X large Jaguar. They responded with yet another variation on the basic MK II/S-type, this time by grafting on a front end styling similar to, but scaled down from, the MK X, onto the S-Type body which was itself an evolution of the MK II using the independent rear suspension developed for the E Type sports car, and the MK X. In August 1966, Jaguar added a new saloon that combined the 4.2 litre powerplant of the Mark X and E-Type with the compact body and independent rear suspension of the S-Type. This was the Jaguar 420. The 420 featured a squared-off nose with four headlamps to improve aesthetic balance compared with the S-Type and to create a family resemblance with the Mark X, (which was renamed "420G" at the same time). All these models carried the by-now familiar twincam XK engine, with model designation reflecting displacement (3.4S, 3.8S, 420).
Typical Jaguar interior appointments were featured, and both manual and automatic transmissions were available. Each version was slightly heavier and less nimble than the one before it. The 420 was capable of 125 mph, and was also sold in England as the badge-engineered Daimler Sovereign with a traditional fluted grille and different trim details. Interior changes seem to have been driven mainly by safety considerations, with the wood cappings on the doors and dashboard being replaced by padded Rexine and only a wooden garnish rail remaining on the tops of the door linings. The adoption of an electric tachometer meant that the cabin clock had to be moved elsewhere, so it was relocated to the centre of the dashboard top. One reason why the 420 is less well known than other Jaguars is that relatively few were made. The factory in Browns Lane, Coventry, stopped making the Jaguar 420 just over two years after production began and with just 10,236 produced.
Although all Jaguar models of the mid to late sixties probably contributed to the evolution of the XJ6, it is probably this model above all which was the closest to the new car in dimensions, and performance. The new (in 1968) XJ6, was smaller than the very large MK X/420G. Several reasons have been put forward for the reason the 420 was produced. One of the more convincing is that it was to fill a market gap between the 3.8S model and the slow selling MK X/420G. Of course Jaguar could have just produced a 4.2 engined "S" type, (and even badge-engineered a Daimler version), but it is interesting to speculate that another reason for producing this short-lived model might have been as a test bed for some of the XJ6 styling and hardware, much of which was similar to the 420. Other sources say that the 420 styling was what the "S" type should have been from the beginning, but lack of time and money kept that model with a MK II styled front end, and that the "S" type therefore possessed unbalanced styling between its MK II derived front and middle section, and its MK X derived rear. The 420 was aimed at the market segment that Jaguar felt the the 420G had missed by being being too large or expensive for the times.
