The 1930s have been described as the era of gimmick-filling pens. This may be somewhat unfair to innovative designs such as Parker's Vacumatic, but the decade nonetheless did see a vogue for new filling mechanisms that clearly appealed to a love of gadgetry. Mabie Todd's flagship pen during this era was the Swan Leverless, which used an elegantly simple twist-filling mechanism, patented in 1932. Unlike earlier twist-fillers such as the A. A. Waterman, the Leverless used a conventional sac, which made servicing as simple as for any conventional lever-filler.
The sac was compressed by being wrung out by an internal entangling bar, shown enlarged above, attached to a knob at the end of the barrel. Turning the knob counterclockwise emptied the sac; turning it back clockwise then allowed the sac to straighten out and fill.
Swan Leverless pens were offered in a wide range of colorful celluloids, and in overlay versions in silver, gold filled, and solid gold. Leverless pens were offered alongside similar Swan lever-fillers, which were, however, significantly less expensive. The Leverless enjoyed enduring popularity, helping fulfill Mabie Todd's advertisement of Swan as "the pen of the British Empire". It seems the Leverless was never offered for sale in the U.S.A., perhaps due to the lingering presence of an American Mabie Todd branch no longer directly affiliated with the main London-based company.
Though Mabie Todd's operations were severely disrupted by bombing during WW2, production resumed after the war with the Leverless still leading the lineup. Postwar Swans were decidedly more sedate: solid colors were the rule, and the Leverless line was given a new streamlined, torpedo-like profile.
By the early to mid-1950s, not only Swans, but also the more economical Blackbirds and Calligraphs began to be offered in twist-filling versions. At the same time, changes were made in the actual mechanism, perhaps spurred by the expiry of the original Leverless patent. The pen above bears a "PAT. APP. FOR" imprint; its filler knob unscrews from the barrel (the metal threads are visible in the picture below), pulling on a triangular loop that in turn pulls the pressure bar down onto the sac.
The other new filling system was essentially the same as a button-filler, but the sprung pressure bar was operated by turning the end knob instead of by direct pressure by a button. This mechanism is most commonly seen on late Blackbirds and Calligraphs. A cutaway model from the Mabie Todd model shop using this basic system appears below, incorporating a filler ring instead of a knob -- a feature never put into regular production pens.