Accumulating a large format setup

[Reverb]

[MartyNL]

What does the silver arm do that I've circled in red?
If it's not possible to separate the back into 2 identifiable pieces in order to remove and rotate the frame 90° then rotation may still be a possibility...

[MartyNL]

Without wishing to insult your intelligence, my newer model rotating 4x5 Toyo back looks ominously like yours...

And my feeling tells me, if it doesn't have a rotation mechanism, then why have a circle on the inside rather than just a square for removal and replace?

[Reverb]

What does the silver arm do that I've circled in red?
If it's not possible to separate the back into 2 identifiable pieces in order to remove and rotate the frame 90° then rotation may still be a possibility...

I have not been able to work that out! Clearly it has some function; I originally assume it was either a locking lever or a lever for separating parts of the film back, either for rotation or removing the glass and fresnel screen. Also, it's about the only part of the back that looks as if it operates something, if you don't count the button that opens the hood. But although it folds out to about 90 degrees, I haven't been able to make anything happen with it yet.

I don't have manuals for any of my large format related stuff: the lenses, the camera, the tripod. In the medium format, SLR, and digital photography world, I have been able to just sit down and work through the manual, and usually everything's explained. LF just seems like an entirely different world, and I've been clueless about all of it. In my defence, I'm a little reluctant to assume what I don't know, and also to use force where it might result in damage. So sometimes, knowing just how hard to press or pull on something to get it to work is difficult. The combination of aged materials and parts that might have seized over time could also have changed the equation from when the gear was new.

[MartyNL]

There are manuals out there as you say. Here is a screenshot from one from Buktus.

[Reverb]

Excellent. Thank you!

The description for the film back is brief: I haven't got it to work with mine yet, but I'll take a close look tomorrow.

[Reverb]

Finally! Got it to move to portrait. It's not very easy at the moment, but maybe it just needs to be worked a few times and it'll get easier. Hope so. Thanks for putting me on the right path. I appreciate it.