Many of the pre X300 Jaguar XJs are listed as "XJ" instead of XJ6. An XJ12 had a 5.3 V12 engine for Series I-III XJs and a 6.0 V12 engine for the XJ40. The XJ12s were the top-of-line versions and considerably rarer than the XJ6s, and people usually call the pre-1994 XJs XJ6s to differentiate them them from the XJ8 which had an all new bodyshell. So how can you tell an XJ12 from an XJ6? Apparently the X300 6 litre V12 had a V12 badge on the pillar, did the earlier models have this?

well on Series I it is possible to tell by the grille - I should look at them and try to identify them onseries II-XJ40 I am not sure if it is possible - that is why we list them simply as XJs

Is it true that with the XJ40 all the cars with the 5.3 V12 were long wheelbase? The thing is in real life people tend to say "Jaguar XJ6" or "XJ8", not "XJ".

You are right ( I am nor sure about the wheelbase), but it is the same thing as noone called S-Class S-Klasse, but S320, S500 etc.. But how to decide without looking under the hood?