a lower thermostat was on my mind as well, but didn't post it for fear of backlash.
after pulling the distribution tube...plan on taking a power washer to the hole several times and seeing if I can get whatever gunk is in there out. 2500 psi pressure washer is sitting in my shed.
retainer looks good with no rust...just dirty. will clean up and paint with rust preventer before putting back in. obviously will reshape.