I'm not convinced that there were tradesmen who specialized in making lashes. Wouldn't they have been made by the same people who made other leather goods for horses, such as bridles, harnesses and saddles? I think more research is needed. It may well have been "last" after all, unless we can come up with some confirmed examples of "lash makers."
I just looked at the entry in question, and the supposed "lash makers" are surrounded (both on his own page and those before and after) by shoemakers and clickers (who made parts of shoes). So last maker would make much more sense.