Problem solved! Well, problem identified, anyway. I had previously posted about some maddening light leaks here:
I ran a test on the back porch: two identical exposures with the sun at my back. The first (attachment #012 below), I shot with my normal technique. The second (#011), following Peter's suggestion in the previous thread, with the dark cloth draped over me & the camera during the entire dark-slide-removal-and-shooting portion of the process.
The former is obviously plagued with light leaks (I sort of baited this problem by standing there for a few seconds with the dark slide out before shooting, but not for an unreasonable length of time.)
So one solution is obvious: just shoot with the dark cloth shielding the back of the camera every time. No problem on calm days, but the setup is sort of unwieldy, especially when making room to get the dark slide out to the side of the camera back, and I can just imagine that it will be a pain on windy days.
So: what tips do you have for ensuring the light-tightness of the camera back & film holder? Or is this a normal condition that I ought to just get used to?
Many thanks for your suggestions!