David,
I've good reports about the metal (but pricey) Calhoon sleds, and also good things about Bob's too. As you said, 7.62x39 wasn't available before, and now it's not even listed as an option.
I messaged the seller of the Ebay sled. He reports he owns a 527 in .17, but has sold plenty of the adapters in 7.62 with no problems. I'll get one and let you know how it works out. It's good that not everyone has forgotten about the 527.
Joe