I have a Plumb Champion around here someplace that is also stamped with a six, just below the etch. It's a large axe and the eye is even oversized, but it wasn't six pounds.
I have also wondered about it. Maybe it's just another symbol to add to the Plumbs?
SYMBOL MARKINGS ON AXES
Axes with a star mark as well as some other symbol markings on some examples suggests they were made for the export and possibly the Australian market. No conclusive evidence is known that verifies that axes with symbols as touch marks were guaranteed to be exported to Australia however numerous axes with such marks have been observed in Australia while considerably fewer have been observed in the US.
Research indicates that more than one system of weights and measures existed in Australia up until the mid 1900s. The fact that the Australians were using more than one system of weights and measures up until the beginnings of WWII and perhaps as late as 1960 may help to explain why Plumb chose to use a symbolic marking system. This was especially true in the early twentieth century and especially for exported axes. No doubt many axes so marked were never exported which helps explain why they are found stateside.
The utilization of designations that included the numbers 3, 3 2, 4, 5, etc. started to be used on axes in North America some time before the turn of the twentieth century and it, like money, may have been recognized before many individuals actually learned to read and especially for those who used axes. In many areas technological developments in Australia took considerably more time. Demands for uniformity became a major consideration at the advent of WWII and they had a major impact on many changes that were introduced starting around 1940.