Harry (Spanner) Rensch and Claud Butler were the pioneers of this type of frame construction method. The Paris Galibier would be just a plain lugless affair without this method. Primarily it was brought about for a variety reasons. Cast lugs were in short supply after World War II. Secondly you can only alter the frame angle on a cast lug by a few degrees, with bilaminates you can build to any angle and still have the appearance of a lugged made frame. We are talking about the period of very short wheel based frames with the crazy steep angles. The lugs with these steep angles were simply not available. Bilaminated construction is not easy and requires considerable skill to make a frame. The bilaminates are cut from flat sheet steel formed around the frame tube and normally silver soldered into place. The majority of the rather ornate lug embellishments on Hetchins frames are indeed bilaminates. Alf and Hymie even had these semi mass produced by having them stamped out. They even tried spot welding them to the main lug to make it quicker for the builder to make a frame. ever ones to cut costs and maximize profits. Regarding cheapness this is a fallacy if you use the 1955 Claud Butler catalogue as your reference you will find that the welded Club is the cheapest next is the lugged and the most expensive is the Bi-Laminated model. This also applies to the Sprint series the bilaminated frame was a whopping £2. 2s. 6d above the lugged version. this was over a half a weeks wage for the working man and would now equate to about £200 extra. Regarding blackboard drawings of lugs I don't know about this, what I do know is I gave Flash some Hetchins engineering drawings of most of their common lug patterns. These belonged to my Father who was given them by one of Hetchins lug cutters sometime in the late 70's. So there were indeed master drawings. H.R.Morris (titch) once told me that he used a shaping machine to cut his patterns for his fancy extensions (stems). Finally we have Harry Rensch, Claud Butler and Stuart Purves (CB Chief Designer) for pioneering this method of frame construction. I hope you will find this helpful and it clarifies some facts, on the other hand you might think its b------s but I can't use this banned word.
Best wishes and be lucky. Michael Butler Huntingdon UK.