Yes, Derek, you were. I typed (carped?) late one night, too knackered to remember tact and common sense, sorry.
Bob Wearing's jig is actually a version he came up with to overcome the problem of gravity slipping the piece away from the cut and the secondary issue of not seeeing the plane cut too clearly, His idea was to invert the wood by 180 degrees and cant the plane instead as Steve says. This allowed the wood to lie flat on the bed of the jig and the operator to see clearly individual cuts as he made them, relative his scribed joint lines.....exactly like the jig YOU built.
What I thought was a D.E. was in fact variously called a 'mitre jack' or 'mitre shooting jack'. Alf ("evenin' darlin'") has one on her site and there are other depictions too.
May woodworm pass you by, all your surfaces be true and your joints be tight.
Sam