Factory Frame rail joining process?

[EmpiGT]

Watching the barndoor build threads they all do something I never saw on later splits. Everyone seems to add an inner strengthener at the seam mid frame. I thought this was just for added strength for the builder's piece of mind. Like this.

Pic borrowed from Artur

But looking thru the gallery this looks like a barndoor only piece. I had 3 early 1956 Wolfsburg built busses without this.

Pic borrowed from Joe Caraway

Why did vw not continue this?
Also in what way did vw weld the frame rails together? If you look at the weld seam itself, it has two peeks with a valley between them running the length of the seam. Not like and arc, mig, tig, or any welded seem I've seen. Someone once told me this was a fusion type seem by putting power to one frame rail and ground the other and pressure. I didn't think that guy knew what he was talking about. But it is a strange looking weld.
If you Google fusion welding it shows nothing like what the vw welded seam looks like.

[Busstom]

I'm pretty sure they're resistance welded. And I suspect they determined the "sistering" plate on earlier frames wasn't adding much value, being centrally located. The floor and crossmembers (and body too) bring a fair amount of stiffness to the frame, but since I am not familiar with barndoor frames, there may be other factors at play also.

https://en.m.wikipedia.org/wiki/Upset_welding

Edit: changed "lapping" to "sistering" to avoid the obvious misnomer. Either way, it's construction jargon

[easy e]

[Okie Adam]

You know, I have this on my April 55 bus and I always thought it was an add on from somewhere in the buses past. I just learned something new!
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55 Wolfsburg Kombi
60 Walkthrough Kombi
62 Single Cab
66 21window
67 so42
76 Westy
64 Karmann Ghia

[npbusguy]

To me, the frame rail welds always looked like larger versions of "upset butt welds" that are used to join band saw blades together. (I used to be in the band saw industry)
For those interested in the machinery and process, here is a link to the foremost supplier of band saw and strip steel welders. https://www.ideal-werk.com/en/products/band-saws/
The process pushes the two pieces of steel together and the slag and impurities form a similar ridge of material thar can be seen on the bus frames.
The same machine also annals the welded area, and then the slag is ground off to create the finished blade surface.
I'd imagine that the machines used to weld the bus frame rails together were similar, albeit a bit larger.