Singer 411g and the mystery problem with the cam stack

I bought this machine last year via Facebook marketplace, recognising the model as one of Singer's best. It's a slant needle model and does chain stitch. In the home of the person I bought it from the machine was ready for me to try out, but it was on the floor; the person was foreign and communication was not all that easy, so after merely pressing the pedal to see if it went, and fiddling with the knobs and levers and presser foot, to check that they moved, I bought it. The only attachments with it were one foot and one special disk. It did have a sticker on it showing that it had been serviced a few years ago.

I then left it for quite a few months as other things had to take priority, plus to be honest I think my doing-up-machines mojo had got up and gone for a while, but once I got it out again the mojo came back. I have since bought two other machines, a Harris Automatic and a Singer 201 treadle in a very nice Enclosed Cabinet No 51, so my case of VSMA (Vintage Sewing Machine Addiction) was clearly just in remission.....

Usual place for machines that I'm cleaning up i.e. the kitchen table - 

I bought a manual from Helen Howes; I know I could have downloaded one for free but this time I wanted a genuine Singer one. I cleaned the machine, oiled it, greased the gears, and tested out the stitches. All was looking good. 

This machine has a stack of cams, which are the metal disks that enable the machine to produce the different stitches. I have a cam stack on the Bernina 801 that I use, but I have never needed to do anything to that, and I have had no experience with working on cam stacks on other machines. I watched this video How to clean the cam stack on Singer 401a - a very similar machine to the 411g but without the ability to do chain stitch, and it seemed a straightforward process so  I went ahead and took mine out and cleaned it. For reasons that will become clear, I just wish I had taken photos of the cam stack before I removed it.

Cam stack in bits before cleaning -

Cam stack after cleaning (we'll come back to the arrows in a minute) -

PIC 1

Cam stack back in the machine -

Now, observe - there is a wiggly spring clip (right hand arrow in pic 1) that should be held in place by that screw (left hand arrow in Pic 2, correctly termed the stud I believe). That screw should  hold down that clip, but on mine it doesn't. It should be sitting right down on it and there shouldn't be that gap under the top of the screw. 

Here is the cam stack from another machine; this is what that screw and clip should look like - 

Someone else's machine

I didn't know this until I went to try out the one special disk that came with it; it worked at first but then popped off the top, meaning the machine was no longer doing that particular stitch.

I asked on the Vintage Singers group on Groups.io (used to be Yahoo) if anybody could help; someone sent me some photos of the top of the cam stack on her machine so I knew mine wasn't right. I contacted Dan Hopgood (good blogger on vintage sewing machines in the UK) knowing he had one of these machines and he very kindly removed the cam stack from his machine, took it apart, gave me measurements I asked for and sent me photos. Everything on his machine seemed to be identical to mine. I took the cam stack in and out, in and out, and took it to bits again and again, but it seems that it is impossible to get that screw down far enough on my machine in order for it to hold that clip in place. Total mystery.

This is why I wish I'd taken pictures before I took the cam stack out, so that I knew whether or not it was like this to start with.

I plan to sell this machine but don't want to do so until I have sorted this mystery out, as even though the stitches that don't involve the special disk work fine I want to be able to sell it with that disk working properly. There are several other disks that go with this machine (how does this happen -  attachments getting separated from machines?!) and some future buyer may well want to to buy extra disks and use them.

So - I'm rather hoping that somewhere out there is someone who will read this post and solve the mystery for me!!