Carl, on 1976 models at least, and probably other years +/- as well, the XJ12 had a thin bright trim at the keyhole level that stretched from the front fender to the quarter panel. It can be seen in the sales brochure. The above picture is a '76 XJ12 on eBay showcasing the trim https://www.ebay.com/itm/1976-Jaguar-XJ12/232525291468?hash=item3623965fcc:g:XXcAAOSwvOxZ5EhS&vxp=mtr
The location of trim just down from the belt line (bottom of the window is the belt line) on a car is known as coach line - I believe it is a traditional name from when there was a horse in front of the buggy! My car currently has gold pinstriping along this coach line.
These are not random drilled holes for pulling a dented body panel straight, these are trim clip mounting points methodically spaced at equal points on a trim line on both quarter panels (same locations on both quarter panels). In my many years in the auto trades, which includes working with collision repair shops, these are the original quarter panels with no obvious points of repair showing through on the back side - save for the filling in of these trim clip holes.
Besides all that, the vin decodes to an XJ12. The XJ12 has an “R” in the vin, whereas the XJ6 has a “T” in the vin. As I’m changing the fuel tanks the XJ12 has a vent line attachment just past mid way up the side of the tank - this plumbing was not on the XJ6. There is more, but with the vin and other clues it all adds up as an XJ12. I don’t know for sure when it was painted, but I do know that Series 3 tail lamps and later year trunk badges are on the car, and later year square outside chrome rear view mirrors too. I actually like the style of the mirrors and the Series 3 tail lamps so all that will stay on the car.
That is my understanding, that the original engine expired. Ever the shopper, the second owner got the SBC long block on sale for $649! It is a 1974 4 bolt main 350 (he paid an extra $50 for a four bolt main engine) originally installed in a police car (code CMJ). It has the requisite Edelbrock Performer intake manifold topped with a Carter 625 performance series carb. HEI distributor, factory rams head exhaust manifolds rather than headers - pretty standard stuff, probably less horsepower but more torque than the V12 it replaced.
Nothing says where the conversion pieces were sourced from, it is a very clean installation and even has Jaguar decals on both valve covers. It retains the Harrison style A6 air conditioning compressor. My non car guy friends didn’t recognize that it was a SBC, it looks right at home.