So, Sekor C lenses, which briefly were plentiful, are now not so much. Still relatively cheap though, except the fish eye and 500mm of course. I looked for a 360mm Sekor C over this last week and only found mislabeled older ones on the 'bay and none at KEH.
OTOH, Pro SD backs are flooding eBay courtesy of Japanese sellers. For a while, you couldn't find these except for relatively expensive rarities and KEH ($), but now there's a crap-ton out there for ~$100 *including shipping* on the 'bay.
I managed to hoard 10 of them over the last few years, wondering when the world was going to run out of these desirable, light-seal-free backs. I guess not any time this week!
Snap'em up if you need one or ten. (I'm not selling any of them btw)