The bike is sort of a steel interpretation of the 26X. If my experiences are telling then it is actually rarer than the 26X as I have seen far fewer in my time. This may relate to low production numbers or the fragility of the frame or both.
They were produced and badged as Airman bikes for Spiegel but were also marketed directly by Monark. In one piece of advertising I have seen, the frame is referred to as the ?Meteor Curve? frame. The two primary variants of the frame are the one on eBay and a similar model with a single top tube that flows past the seat mast and bifurcates into a wishbone above the rear wheel.
Girl?s frames were also produced.
The design of Monark?s twin bar and superframe frames is not as strong as a standard frame and they tend to distort and break in situations that would not noticeably harm a normal bike. The meteor curve frame is weak in all the same areas plus the rear dropouts have no triangular bracing to the seat mast and tend to break at the webbing for the wheel set screws. Both sides of my frame have aftermarket welds in those locations to hold the frame together and I believe I noticed a similar repair on the eBay bike when I looked at the photos last time it was on auction.
I have seen advertisement for the twin-bar models showing it both equipped with a tank and without. I have only seen the single top tube model shown with a tank.
The tank used is the same hanging tank used on the superframe five-bar but the top tubes are flatter than the curve of the top tube on the superframe. To fill the dead airspace between the tank and the frame a triangular piece of metal was fitted but no one I know has seen an original filler piece to know exactly how it was shaped or formed and how it was attached.
1940 Airman model literature can be found on Dave Stromberger?s Nostalgic.net website