I haven't heard of a Martin made stencil called Lewin Bros. London. But Martin made saxes for lots of companies. When it comes to stencil saxes, it's like Kevgermany writes, the prices are often lower compared to an original sax. But a good F.E. Olds stencil made by Martin goes for almost the same money as a Martin from the same time. They were almost as good as the Martin. F.E.Olds also had their own design on the keys, braces .. . A sought after model is the ”Super”. Other stencilsaxes could be made out of old inventory (tubes, keys...). I think the costumers (brandholders) got what they paid for.
There must be a player who appreciate an old sax. I think a sax from the 20's has less playability than a younger or modern sax. Hard to find a mouthpiece, worse ergonomics and keyworks, differnt tone/sound .... . But if you're playing jazz from the 20's or classic saxmusic it might be the right sax for you.
When we are talking about stencils we should also talk about the second line saxes like "Indiana" made by Marin Band Company". Indiana was owned by Martin but they had thier own market and sales department. They were also made in the same plant in Elkhart. The saxes were sold as a lowprice alternative for schools, marching bands, concert bands …. . My Indiana tenor -59 have keys like my Martin HC Comm I from -38. But not nickelsilver!! So I guess when the tools that made the saxes were worn out to produce a pro-sax they extended the lifecycle by using them to produce a secondline sax. There were less quality control as well. Very good Indianas reached the market/players. But so did also the bad ones. I think they didn't play or fitted the keys before the saxes leaved the plant. It was cheaper to replace the bad sax with a new one. An Indiana sax can be as good as a real Martin when it comes to intonation and tone.
Thomas