I have a couple of 6x9 Folders:
Franka "Rolfix Jr." (Made in Germany - US Zone), with which came with the 6x6 mask. VF has frame lines for 6x6 & 6x9.
f:4.5 Anastigmat Frankar 105mm, uncoated. scale focus.
Vario Shutter: B - 25 - 50 - 200
Hyperfocal marks for "point & shoot" : 13 ft @ f: 11
Voigtlander Bessa - pre-War ? f: 3.5 Skopar 10,5 cm lens (uncoated), Compur-Rapid Shutter: B - 1 - 2 - 5 - 10 - 25 - 50 - 100 - 200 - 400
Originally had a 6x6 mask (now missing), and the Viewfinder has a flip-up 6x6 mask. Wondeful camera, big, crisp lens.
HF marks: f:8 @ 11ft for close subjects ( 8 - 16 ft);
35 ft for distant subjects (20 ft to Inf ).
My other "nice" folder is a Zeiss "Ikonta 521", which is 4.5 x 6 cm. It has the 75mm Novar (coated) in a Prontor shutter. This is a tiny camera, and will give 16 exposures on a roll of 120.
The shutter is sticky due to oil on the blades.
One other beast that I shoot 6x9 with is a late 1920's Zeiss "Donata" 9x12 plate camera, equipped with a 120 roll-film back.
It's hard to go wrong with a Zeiss camera; RF -equipped models generally cost more than their non-RF cousins.
Good luck !
Luddite Frank