The camera is actually equipped with four windows on the back, one pair in red for orthochromatic film and one pair in green for panchromatic. Those distinctions are no longer of importance in using modern films, and because they are considerably more sensitive than the emulsions available in the 1930's it is prudent to keep the windows covered except when advancing the film.
The Foth Derby, in spite of its classic design and construction, can often be found on ebay selling for as little as ten dollars. The reason for this is that the cloth shutter curtains in the camera are always severely deteriorated. This can be verified by holding the open back of the camera up to a bright light and looking in through the lens. What is revealed is a field of tiny pinholes where the rubberized covering has worn away from the cloth base of the shutter curtains. Fortunately, the deterioration can often be corrected by recoating the curtains with a layer of opaque ebony matte fabric paint, obtained in the U.S. in craft stores under the brand name, Tulip.
There are, however, some other things that go wrong with these old shutters. The tension on the curtains may become uneaven so that they do not travel at the same speed to maintain the proper gap during exposure at each setting. The ribbons holding the curtains in place can come loose and do not hold the two curtains in proper alignment, thus permitting a fatal gap to appear during film advance. If you are an adventurous repairman, some directions are available on line from RaulM for shutter restoration in the worst-case scenario.
A web search on 127 film will still turn up a few remaining retail sources; the prices run the gamut from exorbitant to ridiculous. Similarly, one can find a vast number of strategies for rolling your own 127, but all are at least somewhat labor intensive. Many will ask, "why bother". One answer is that classic camera manufacturers produced a great quantity of exquisitely designed 127-format cameras, of which the Foth Derby is but one example.
The Foth Derby Manual is at the Butkus site.
More pictures made with my Foth Derby may be viewed in a folder at Flicker.