My understanding is that the wide license plate opening is not a "Euro" vs "US" issue. All Panteras built until 1978 had the narrow opening, because DeTomaso was still using left over stampings from the Ford era. When those ran out, the new rear clips were all then made with the new wide opening. George Pence wrote an excellent summary of this history previously here on the PIBB:
"...while Ford built Panteras they were contractually obligated to supply Panteras to deTomaso for the European market. Ford & the deTomasos parted company on Sept. 8, 1973. Ford stopped producing Panteras around July 1974, the last Pantera shipped to the US was chassis number 7380. Ford closed the deTomaso Automobili factory and sold the remaining unfinished coaches to Alejandro deTomaso, who opened another factory under the name deTomaso Modena and continued assembling GTS Panteras, at a much slower pace, using the coaches he purchased from Ford. Those cars have chassis numbers 7501 to <7700.
As the coaches purchased from Ford ran out (about 1976), deTomaso contracted Carozzeria Maggiora to manufacture GTS Pantera coaches. The relationship between Carozzeria Maggiora & deTomaso lasted less than 2 years. Those cars have chassis numbers 9001 to <9099.
After the relationship with Carozzeria Maggiora terminated, deTomaso contracted Carozzeria Embo in 1978 to manufacture Pantera coaches. Carozzeria Embo manufactured all the remaining Pantera coaches, including those of the Pantera Si. The coaches manufactured by Carozzeria Embo have chassis numbers 9101 to 9562, and 9601 to 9641 for the Pantera Si. For the first 2 years, Carozzeria Embo only produced GTS Panteras.
All the GTS, GT5 & GT5-S coaches manufactured by Carozzeria Embo have the squared off rear wheel houses.
your friend on the PIBB, George"
Charlie McCall has also posted valuable information on this subject:
"...regarding the squared-off engine bay... When Ford and DeTomaso divorced in '73, Ford, in a moment of spite, destroyed the tooling required to stamp out the car, figuring that DeTomaso wouldn't be able to continue manufacturing cars without the tooling. So instead of stamping out that area in one large die, they had to fabricate the area, welding panels together, more or less. Hence the straight, welded panels in the later cars. Again, it isn't model-specific, but instead when Alejandro ran out of stamped panels, everything from that point on was manufactured, regardless of model."
I admit to some confusion on my part about the Maggiora period, because most of the cars for which I have entries in the registry with chassis numbers from #9000 to #9100 have model years from 1978 to 1980, not 1976-1978 (the dates both George Pence and Franz Krump agree were Maggiora). The cars with chassis numbers higher than that are 1980 or newer model years.
In short, I believe it may have been both the Carozzeria Maggiora and Carozzeria Embo coaches that had the wide license plate openings, and it was certainly all the Embo cars. The one thing I am pretty sure about is that, based on the cars I have in the registry, after 1978 (chassis #9001 to #9641) all Panteras had the wide license plate opening.