Thanks guys ... and a big thanks for whunter for digging out and sending two of them for me, even if they ended up not fitting! The part number I got off the side of mine, which is very hard to read so I did a rubbing of it, looks like 6172031473 ... however, the EPC lists two other numbers (I will bring them up when I get home from work later). I might give it another go flattening out the surface. The funny thing is, it doesn't look that corroded ... it had one uneven spot that I tried to even out using JB Weld ... but maybe it's actually warped? I'm also still not convinced I'm putting the gasket sealer on correctly, but I don't think it should necessarily even need the sealant anyway, correct? I could probably try another round of JB Weld, flatten with a scraper and then sand with a flat block. I should have ordered about 20 of the paper gaskets, because I keep wasting them, heh.
Here are more pictures of mine: Tstat housing - a set on Flickr

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)