An update on the Yashica M:
With the help of Funky Gene and a video he recorded about this camera, I was able to get the film into the camera. It just took more force than I'd been comfortable with applying to a 65-year-old camera that I know nothing about.
And then another problem reared its ugly head. About 5 frames (plus a couple-three wind-in frames) into a new roll of Tri-X, the film jumped the tracks and jammed the advance. I tried to save the rest of the roll, by pulling the exposed film out of the takeup spool and winding it back into the cartridge. Which seemed to work, but the film did the same thing at about the same place, maybe a couple of frames further in.
I was puzzled. But then I realized that the rewind crank, which on this camera is weirdly on the bottom, had not been locked beck into its recess. This would leave the rewind spindle connected to the crank... which can't be conducive to proper film advancement, neh?
So I popped in the roll of expired-in-2010 Kodak UltraMax 400 that dude gave me, and we'll see how it goes. (In retrospect, I should have used it first, but I didn't want to use 15 year old color film as a test roll for the camera. That was an 11 dollar mistake... If you get a free roll of film with an old camera, use it first.
--Shannon