My car is a 91. The firewall plate on the US passenger side is the same from the pictures I have been seeing. I wouldn't mind having this verified for sure though. If you happen to have a photo.
Here's a picture stolen from @carnut222 's thread, of his 79 wagon.
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I did a bit of digging in the parts books. In Volvo parlance, each black firewall plate is called a hatch. The LHD and RHD hatches do bear different part numbers.
I'm unsure if this is because they're pressed differently, or if they're just drilled differently (LHD cars are a bit thin on the ground here to compare them visually...). So I'm going by the numbers in the book. For example, p/n 1246235-4 appears to have a 17.5mm hole for fitting the slidey-lever-type UCOMBU heater box, which we didn't see a lot of in OZ.
Here's a list of hatches - many of which carried through from the 1979 year model to the end of production.
left, lhd = 1382106-1 [1979 on]
left, rhd = 1247706-8 [1979 on]
right, lhd = 1246235-4 [1979]
right, lhd = 1315028-9 [-1990 option] [std in 1985-7] [1979-83 = LHD INCL B21F CALIFORNIA, B21F (J) B27F,B28F. (1304991)]
right, lhd = 1382461-0 1991-, no abs [was 1382408]
right, lhd = 1382461-0 1991-, abs [was 1382414]
right, rhd = 1382113-7 [was 1255566] [1304987 alt superceded in 1985-7] [1979-84 std]