The line throwing guns were similar but were marked Model 270.
The Model 209/277 were the shorter barrel gas guns. They had revolver grips. There was also a variant with a wooden shoulder stock and a longer barrel, the Model 209/276.
I think they were first made in the 1960's. The Standard Catalog of Smith & Wesson mentions them but doesn't say when they were produced. S&W sold them to a variety of state, county and local police departments across the country.
I've got a line thrower and a gas gun, the long barrel one, which came to me with a pistol grip. I bought a wood stock for it from a member here but haven't found the correct mounting hardware and I don't think Home Depot or Lowes would appreciate me bringing it into their hardware aisle and trying screws out.
They are kind of cool.
