Polybus, what jonesy posted was a series of 20 year-letters, so every 20 years the series would start again. As I read it, that means 6F is either 1937 or 1957, not somewhere between. There should be enough major design differences, aside from the known production period of the Model 6, to make it easy to tell which it is. Not that I could tell, but I'm pretty sure jonesy could.
