The International Cessna^® 170 Association

I've seen stains like this near the throttle in several pictures online. What causes it?

I've never actually been frisky enough to take that plexiglas panel off to even see how it's attached. I know it's got a couple of hidden screws, but I'm guessing the engine controls hold it in place rather than it being glued down. If it's not glued (and I suspect they're not) it would be reasonable to assume that water could condensate between the main panel and the plexiglas overlay. If it has no efficient method to drain, it could probably get pretty funky over the course of 60 years.

The plexi panel is attached by the control cables that go thru it. Pretty sure there are no extra screws.
There are flush screws UNDER the plexi panel that hold on the piano key bracket behind the panel tho. Christine, if you look below the throttle, you will see where I drilled the plexi to access these screws when I rebuild the piano key assembly (to avoid pulling every single control cable out of the airplane)

The back of the plexi is painted with the design and lettering. I think what you're seeing is condensation that has eaten into the paint and faded it.

Removing the plexi is a bear of a job because the control cables go thru it and have to be completely removed.

bagarre wrote: There are flush screws UNDER the plexi panel that hold on the piano key bracket behind the panel tho.

Those may be what I was referencing. I recall seeing some that were concealed underneath the lower quarter trim panels, but I was thinking that they went through the plexiglas. I didn't disassemble quite to the level that you probably did with your re-wire, David, so you're likely more familiar. I was ready to get the airplane back together ASAP...pain in the ass working on that kind of stuff.

Both. And they are all to hold in the piano key assembly.

Yellow lines point to the accessible ones that you saw that go thru the plexi.
White lines point to the ones under the plexi that I drilled to get access to.
I still don't know how I landed two holes directly on top of the screws as you can't see them at all.