Hey Momus, it's great to find another Argus 40 user! Well, technically mine is labeled "Argoflex Forty", but it is the same camera.
Believe it or not, I know exactly what you are talking about with the extra space on the end of the take up spool since mine is like that too. The gap isn't especially large, but as maybe you discovered too, it is enough that the film will drift over towards that side when it is wound on, and that results in the exposed frames being shifted over close to the one edge of the film. As far as I know, this is not due to any missing piece, but rather is just the camera design.
Anyway, the spacing issue bothered me enough that I decided to make a simple fix. Basically, I just cut a small piece of plastic to use as a spacer, and glued it in place over the slanted rib on that side. Here is what it looked like before the mod:
This is a shot of the piece I made:
It is about 1mm thick if I remember right, and was cut from a sheet of black styrene (I'm sure there are a variety of other materials that would work just as well). For glue I think I decided on Super Glue (cyoanacrylate). You might notice that I beveled the leading edge with a file so that the spool could still be inserted smoothly without catching on the edge when inserted.
And, here this is the modified result:
I've tested this now and it seems to work just fine!
By the way, you might also know me as Jeff Applegate over on the Photo.net forums, or on the Yahoo Argus group too, where I recently joined.
Jeff