The Western hatchet posted by the OP is a model number F10.
The model number of Mass Massoni's hatchet is the F14.
The knife/hatchet combos were the F6610, F3910, F6614 and F3914. The 66 and 39 referred to the knife models in the combos.
The knives and hatchets were also available individually.
The F14s never had a model number on them, even though they were manufactured long after Western started putting model numbers on knives. I believe this was due to the heavy duty construction of the handle. The F10 had a lightweight, knife-like ricasso area that just "asked for" a model number on it. In fact, the Model 10 hatchets (L10, W10, F10, R10) were marketed as "lightweight axes". The F14 was called a "Camp Axe". Apparently, there was never a model L14 or W14 made. At least I've never seen one in 40 years of collecting them.
Western began putting model numbers on knives some time between 1952 and 1955. In the book The Knife Makers Who Went West, Western President Harvey Platts states that Western dropped reference to the double tang patent around 1952 and also that they started putting model numbers on knives around 1955. However, I have knives with both the patent number AND a model number.