Additonally, early barrels had smaller lugs for the breech block.
Oldest to Newest No barrel ribs Barrel Ribs
Left:12F iron with ribs. Right: 11F iron without ribs.
The earliest cannons did not have a rib between the sump and the barrel.
Later castings had this rib between the rib and barrel.
As an amature foundry man, I believe the ribs were added to prevent casting flaws
or to make the pattern easier to draw from the sand.
Barrels without ribs are older, but it isn't exactly know when this change took place.
The 6F has three different versions. The earliest have no rib on top of the barrel.
The rib-less version is seen in black and khaki.
The 6F production overlapped the early breach loading type and the later charger type cannons.
Because the early cannons were black and the later ones were khaki, it is reasonable to assume that
the black ones were 1929 to possibly 1935. And the khaki ones began around 1935 when the charger
cannons were introduced.
Since the early 6F's had no rib (the barrels were ground top and bottom at the parting line
just like the other breech loaders, this practice probably didn't stop right when the charger cannons
were introduced in 1935.
The charger cannons have a distinct rib at the parting line and this makes the casting easier to clean up.
At some point, someone must have suggested doing the same thing to the 6F.
Therefore khaki 6F's with no rib are most likely 1935 or later.
6F's and 60MM's can be divided into two versions.
The earlier version has steps in the top rib where the large barrel diameter comes down the the final tapered length.
The later, and current, version does not have these steps, but has a smooth ramp.