I see-- I'm in the 150-160 range, so perhaps I'll add a new B66 to my list instead. It's cheaper and more appropriate for period work up anyway. I set my bars a good 2-3 inches higher than the saddle and ride pretty upright.

As for the B-73-- it's the only saddle I see produced new with 3 coil springs. I suppose they were more common many years ago, but given how coil springs on saddles work it would seem you're right about that lateral motion issue.