Originally Posted by Jeff Wills

I remember clamps like that on BMX bikes, including the fat-tubed Schwinn Sting. The QR is a tad weird- but plenty of early MTB'ers had QR seats to lower them for downhilling.

I got this reply from Jim Langley:
"I'm thinking it might be an early bike by Tom Ritchey or built by Tom for Gary Fisher. They used those press in bearing BBs and TA cranks and Mafacs. The fork crown is a bit strange, but not unseen on mountain bikes."