They broke the piece that screws into this positioning bar.
They then tried to silver solder it and messed up the little disc.

This is just another view of the bar showing the broken piece inside.

Lets start on the repair.
After thinking about it, I decided to make the broken piece in two pieces. I started by threading a piece of stainless steel.

Why Stainless Steel - You Ask...
It's tougher than brass and this is one of the weak spots in the design.
Gillette changed the design in the next version.

The next step was to make the disc. This I turned out of brass. It's only there to hold the big spring in place and there is no force on it.

Here I'm just checking that the threads match -
Yes - It's called OCD 😜

The top half of the image above shows the parted disc with the thread and the bottom shows both pieces silver soldered together and it all cleaned up.

Up to this point, I hadn't touched any part of the razor and I contacted the collector to confirm that I could go ahead - which of course he agreed!!!
Its time to drill and tap the positioning bar.

Here I'm done with tapping the shaft of the bar.

I'm just testing for depth. I left the top thread long in order to get it to fit deep in the center bar.

The finished piece in the bar.
It's long and because its made of stainless steel it far stronger than the original piece.

Everything together.
At this point it can be put back together and it will work fine.
The Adjuster dial does not click and in order to see why, I need to take the head section apart.

Checking everything again before I move onto the next part.
Yes call it my OCD again 😜

The remade piece.
This should be the last time that this piece gets put together.

So far so good.
I'm extremely happy with the progress so far.
Let's move onto the second part of the repair.

Current status of the head section.
It's soaking in a citric acid solution in order to try and loosen up the soap scum.
UPDATE - This cleaned the soap scum away but didn't do anything to loosen the piece that I need to pull off.
I need to move to Plan B.

Plan B: I have the head sitting in my kitchen with a spray can of penetrating oil next to it.

Plan B Worked 💪
This is what has come apart.
Now for the bad news..... The little square spring is broken 🤒
I currently have the pieces soaking in the citric acid again.
BTW - On the left is the nylon piece that I turned in order to tap it apart.

After some thinking....
I found the round piece of Stainless shim stock at work and it felt springy. I cut a small piece of it and tried to bend it, but it broke.
Onto Plan B. I thought of a DE blade but when we bend them they also break.

This is Plan C.
I started with a Single Edge Injector blade.
Then cut off an appropriate size.
Some more trimming, "bluntening" of the edge and rounding of corners.
The final step is to bend it in the middle.

ALL DONE!!! 💪
All the parts.
Its now ready for the final assembly.

It's all together and everything works as it should.
It opens and closes as it should and the Gillette "Click" is back.
There was some more fiddling, sandpapering and trimming to get the spring into the slot, but I got there in the end.

So how does it shave?
More or less like a FatBoy - it's mild.
I think that mine is slightly more aggressive and to really tell, I would need to use both in the same shave, however this needs to get back to its's owner.

To finish off.
Serial Toggle 2906 back in service.

I'm truly honoured that I've been able to restore two Serial Toggles.

They have a very delicate mechanism and after these two repairs, I fully understand why Gillette changed the design.

The weak spot is the connector bar that joins the Toggle to the positioning bar. It's made of brass and will eventually break. It would have been much better made in Stainless Steel.