As I thought, Lens ID is 142 for them all.
What I found is that in darktable it was searching for the name Canon EF 300mm f/2.8L IS
I changed slr-canon.xml from EF300 f/2.8L IS USM to Canon EF 300mm f/2.8L IS and that one works now. So it reads LensType and not LensModel.
x1.4 II and x2 III worked before but now none of the three converters are identified.
C00 files below
300.CR2 (29.4 MB)
300x1_4II.CR2 (29.0 MB)
300x1_4III.CR2 (29.7 MB)
300x2III.CR2 (27.8 MB)
slr-canon.zip (59.6 KB)