As far as I know, LensModel is not stored as a string, but rather, as a numeric code, which gets encoded/decoded by the tool which is reading/modifying the EXIF data (exiftool or other). There are no EXIF codes for FD lenses.
I have a vague memory of finding another EXIF field that stored some lens information that could be written as an arbitrary string, but I can't find my notes right now. Sorry.
If you do recall, I hope you'll post it here. I've tried -LensModel, -Lens and -LensID and none work. I don't know if there are others; I can't seem to locate such information.