Interesting, I didn't know these even existed! It would be fun to compare performance with the 1GHz sonnet ZIF upgrade. Two 7410 chips at 600MHz should beat it in many cases, at least with OS X. Doesn't the sonnet use a 50MHz bus instead of 67? Anyway, I found it here:
www.ebay.com/itm/374150035863
...and searched the forum for info. This old thread was all I found, so I thought I'd bump it and steal some pictures from that link above to preserve them:
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It looks like there were at least two versions made (the board doesn't match the picture on the box). For searching, it was called xlr8 MACh Velocity MPe for ZIF sockets G4 or G3 beige, etc.