@Dick Steinkamp Thank you for providing a photo of your 3.0 Bav armrest. Measurements are the same as my e9 armrest.
I think the possible confusion comes from there being two Schwarz e9 part numbers, one for 2800CS (51 41 1 804 975) and one for 3.0CS (51 41 1 815 283) but the actual armrests are the same. This also applies to the e3, there is one part number for 2500/2800/3.0 Bav (51 41 1 804 975 and 51 41 1 806 927 for later 2500/2800 and 51 41 1 813 621 for 3.0 Bav with electric windows only) and another for 3.0S/3.0Si/3.3 (51 41 1 815 283) but armrests are again the same. Perhaps the difference is the shade of Schwarz but who knows. Also, we know some have used the chrome plastic baseplate for the 3.0S/Si armrest on their e9 and it does fit.