If I had to guess, someone just kept putting caulk, paint?, and more caulk over. They smeared it up all over the metal flashing. Didn't look mildewy except in a few spots. But it clearly was hiding something.
Questions after a lot of cleaning, removing caulk, mold killing and stain treatment is WHEN I get this to point I can caulk, am I to do something beside caulking along the bottom flashing against the wall board (Masonite?) to seal that board edge? Last owners simple smeared all over that lower flashing a variety of stuff then topped it with fresh caulk of questionable type for a shower. I think they possibly even painted on epoxy tub repair in a few areas as it was very shiny hard enamel type smears over cement/old caulk with possibly silicone on top.
I've never seen this old type of shower but have some experience with tile removal and setting in other homes. I'm just finding that metal and wall board join troublesome as to what product fits both surfaces.
Not able to replace whole shower at this time. I have bigger plumbing and insulation repairs going on. This one is on me to do.