dumb rx01 image question

[I sent this on monday but it seems to have been lost; sorry if it's a dup] I have a dumb question about rx01 image files used by simh. I used "copflp" (by John Wilson - thanks John!) to image a box of rx01 floppies. I then put the images on pc and used simh and I can see them fine with rt11 running under simh (i.e. "att rx0 image.dsk") But an old program I have which will read rt11 images does not see the directories. I am confused because it works fine with mscp image files that I connect to simh via an ra device (like an rd32). What's the difference between rt11 ra image files and rt11 rx01 image files? I would think the rt11 volume and directory info would be in the same place (but it seems I'm wrong). I tried skipping track 0 by offsetting 128*26 bytes, but that did not help. It looks like 128*25 might be a better offset for floppies. what am I missing? (and thanks in advance!) -brad