As far as the lights on coupes being different, theside 'repeater' lights on XA's and early XB's were vertical, later XB's and XC's had them horizontal. As has been stated the XA coupe lights are recessed into the back (slope inward), the XB lights were a flat panel with a chrome surround and the XC's had the same but the surround was black. Someone may be able to correct me on this, but I don't know of any XC's with the chrome unless they were bought after market.
The floor pan stampings are different. They changed during the XB range. The strengthening ribs are in different locations.
Window wipers had a different mounting system. Xa and early XB had large spline and clip, late XB and XC had smaller spline and retaining nut. I found this out the hard way buying a pair on Ebay, ended up with early XB when I needed later ones.
Points above relate to coupes, but it's possible pans and wipers could have similar differences on sedans
I'd suggest looking at project threads for clarification.
Look at Dean, Recoil and Daveman threads for XA's.
Danthefordman, Turbocoupe, Tippy and mine for XB's
Buggerlugs for XC
Or if you want to be confused, Peachey has an XA with the XC front on it
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Hope this helps!