Ooops this might get messier than anticipated. Full list of XJS codes (deep breath ...) from ARonline is:

XJ27 XJ-S Coupe
XJ28 XJ-S Convertible
XJ41 F-type Coupe
XJ42 F-type Cabriolet
XJ57 XJ-S six-cylinder Coupe
XJ58 XJ-S six-cylinder Cabriolet
XJ63 XJ57 with projected Getrag manual gearbox
XJ71 XJ-S mules for XJ41 development programme
XJ77 XJ-S V12 Convertible
XJ78 XJ-S AJ6 Convertible
XJ79 4WD XJ-S
XJ87 facelift V12 XJS Coupe
XJ88 facelift AJ6 XJS Coupe
XJ89 facelift XJS Cabriolet
XJ97 facelift V12 XJS Convertible
XJ98 facelift V12 AJ6 Coupe

.. which has several dead-ends, non-production experiments, and too many things which are visually indistinguishable, so it's unusable for our needs.

XJ77 was the 1988+ V12 full convertible, which we've stretched by usage to include the subsequent AJ6 4.0 version launched with the 91+ facelift - fair enough, it gives us an identifiable collection for non-coupes. Splitting off the 4.0 wouldn't work in practice - the only clue is the bigger bonnet hump for the smaller engine, but Jag started putting the bigger hump on the V12s as part of the 91+ facelift, which means it becomes a dating attribute across all convertibles, not an engine marker. So all full factory convertibles being XJ77 is all we can do. And its mirror for same reasons of all coupes being XJ27 is similarly practical. The American Hess & Eisenhardt convertible conversions of XJ27 coupes predate factory XJ77, so XJ27 works fine for these without change.

XJ28 convertible in the above list was pre-prodction 1970s code for abandoned experimental/prototype only, but may have been revived for 1985+ V12 SC targa (see below).

XJ57/58. These are the two 3.6 six cylinder twins launched Aug 83, but not the 4.0 AJ6 sequels. Hopeless task to try to incorporate XJ57 for the coupe, as the humped bonnet is the only visual clue and it's difficult to use in practice when visible - I've tried, with some results but not enough to fully trust. And then all versions get it .. XJ58 was only used on the 3.6 SC targa convertibles (there were no 4.0 SC/targas), but digging deeper I discovered there were V12 SCs as well (July 85+) so although SCs are distinctly identifiable they're difficult to split (only that bonnet again), and there's no distinct code in that list which securely says V12 SC (unless it was revived for the V12 SC which is unproven and nobody ever uses it).

Summary - stick to what we've got, and abandon XJ58 idea - it's not reliable enough.

On the bright side, we (fingers crossed) seem to have successfully incorporated the stupidly trivial 1991 name change from XJ-S to XJS across everything.