This Zieleman mountian frame shows fluted tubing in front triangle (what manufacturer?). Some frame parts like the bridges between the stays I only recall from seeing on frames of manufacturers with their roots in road bikes.

Fork blades are brazed into a subtle crown/lug.

Geometry is traditional early style MTB. Level top tube. With eyelets on fork and frame the Zieleman is ready for fenders and racks.

I personally know one other Zieleman mountain frame. That one is in the stable of a fellow collector who rescued it out of the hands of a bike-polo kid.