Above, a very interesting 90 cm mahogany level marked with the FM Fornhem Møbelkunst logo. It has plexiglass coverings on the spirit vials, and unusual hardware on each end. It also has two enlongated holes in the body to assist in holding it.
"Fornhem Møbelkunst" is Danish for "prestigious furniture art." Coincidentally (or not) the "FM" apparently stands for "Feldballes Møbelfabrik."
According to decopedia:
Feldballes Møbelfabrik was established in Århus, Denmark and was active in the 1950s. One of the highlights of its production was based on the cooperation with furniture designer Kai Kristiansen - the so-called FM system, or "FM-reolen". This elegant and highly flexible shelving system was made in teak, oak or rosewood.
Kristiansen's system featured slotted metal wall standards to support the movable shelf brackets. I suppose that the unusual hardware on each end of the level was intended to hook into these standards to assist in ensuring that the shelves were level.