Big-Bang Model 6F
Introduced in 1929, the Big-Bang model 6F has been in continuous production except for a brief period during WWII and again in 1952 when the factory burned. It is estimated over five million have been made. This toy has passed its 70th birthday and is possibly the only American iron toy to survive the depression. The Model 6F currently sells today for $69.95 from Conestoga, but you can get it for less from Flash60 LLC.
This is a type-A 6F. There is no rib on top of the barrel. This style is believed to have been discontinued about 1935. Type-A 6F's have been observed with both khaki and black barrels. The box is the most colorful 6F box and is refered to among collectors as the "bullseye" box. Type-B 6F's are know to have been marketed in the bullseye box.
This Khaki 6F was made between 1956 and 1958, the period when Big-Bang was Commended by Parent's Magazine (tag on box). This is a type-B 6F. The top rib has distinctive steps where the barrel diameter changes. Type-C (not shown) has a ramp in the rib instead of the steps.
Dave Ross found what might be the earliest version of the 6F with a horizontal bar in the barrel just ahead of the water sump. The breech block is of the early type as seen on the wooden army tank.