I'm trying to patch together a couple of adjacent MSS images, prior to
rectifying the patched group. The only guides I can find about figuring
the overlap between the two are not terribly helpful: "about 10%".
Is there some straightforward way I can work out what the relationship
in x,y pixels is between these images?
I would suggest you display your first image, then overlay your second image,
and then use d.what.rast to determine the exact overlapping pixels before
re-extracting only the desired rows of your second image.