Need help on 50mm A f/2 with stuck focussing

[Peter Spiro]

I was rummaging in my local camera store, and found a Pentax A 50mm f/2 at a 
bargain price.  The "only" thing wrong with it was that the focussing ring 
wouldn't turn.  I thought it should be an interesting challenge to try to 
fix it.   (From previous experience with A and F lenses, I knew I shouldn't 
remove the aperture ring, since tiny parts pop out which are almost 
impossible to replace.)

I have succeeded in pretty thoroughly taking it apart.  The toughest part is 
often getting the front retaining ring off.  I got a roll of electrical tape 
that was the same size, which I pressed down on it, and I managed to unscrew 
it fairly easily, without damaging it.  After that, it is a simple series of 
small screws and retaining rings, and all the front parts come off pretty 
easily right down to where the diaphragm is.  This is a picture of what it 
looks like disassembled:

http://ca.geocities.com/spirope/pentax50aparts.jpg
Well, now I have it taken apart (and I took digital pictures of it along the 
way, so I would remember what goes where).  However, I have no idea what to 
fix now.  The focussing ring is now loose, and turns nicely, but it doesn't 
engage with anything, and even worse I can't see anything for it to engage 
with.

I would have thought focussing is a pretty simple matter of one object 
turning against the threads of another.  I thought perhaps the parts had 
gotten misaligned, and it would be a matter of snapping it back into place.  
However, I can't see anything like that here.   Does anybody know what the 
moving part is supposed to be for focussing this lens?

I don't have another 50 f/2, so I don't know if the rear element is supposed 
to move when it focusses.   Does it?  Any tips would be appreciated.