The instrument in question was designed to be able to dialogue both 8’ registers on separate manuals.  The OP (to whom this was communicated off-list) was correct that the upper manual does not slide to couple.  The lower manual slides to couple.  The lower portion of the dogleg jack engages a clothed block on the lower manual.   To uncouple the upper manual dogleg jacks from the lower manual is the work of pulling the lower manual toward the player, thus avoiding conflict with the middle rank of jacks.  To couple, one pushes back.  This is a direct crib from the originals and provides the lightest action.

Best,

Hendrik 

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