Front 8' jacks shared on both manuals via a dogleg system was actually quite common right up to the end of the 18th century. the best known examples were English doubles which as far as I know never had manual couplers. 18CFrench harpsichords generally had manual couplers, on the other hand many 17C french harpsichords had doglegs or nothing at all and many german harpsichords had doglegs instead of manual couplers. 

The advantage of a dogleg system is a lighter action on the lower manual, the disadvantage is that you can't dialogue separately the two 8's one manual to another.

Richard

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