When Multigrade first came out in the seventies (I think it may have been older than that, but that's roughly when I encountered it for the first time), there were two filter kits available. One that went above the lens, in the neg carrier, and for older enlargers, there was a below the lens kit. This was a holder with a circular aperture above a filter rest, and with three screws in the same plane, which held the filter mount on to the enlarging lens itself. In theory this would have resulted in some image degradation, compared with the above the lens kit, but in practice, I never noticed any. I'd an old Gnome Beta II enlarger, with a Nikon EL lens on it, and it worked a treat. I still have it, along with the full set of filters, read for the day when the darkroom is back in service.