Canal Streets were single spring. Might be hard to track down though.
Queen also made single spring muskrats (as well as 2 spring). Keep in mind that Queen also manufactured muskrats (and other knives) under the Schatt & Morgan and the Robeson names.
Case made the 2004 Tony Bose muskrat as a single spring, but this is a Hawbaker, and I don't know if you're interested in that blade configuration. The regular production Case muskrats are 2 spring.
On the subject of Hawbakers, Canal Street made some in that blade configuration, as well as Queen, including the S&M and Robeson lines.
All the above are no longer in production. Queen has recently closed its doors (at least, for now), Canal Street a few years back, and the Case/Bose was only in 2004.
The Schrade 77OT and Camillus 714 are 2 spring.
I can't help you much with the German brands, except to say that my Frank Buster Fightn' Rooster is 2 springs.
I would say that your best bet for a great knife is Canal Street, even though the company is kaput. The regular muskrats weren't terribly popular, so when they pop up for sale they still go for reasonable prices even though the company is defunct (the Hawbaker will be trickier).