There are multiple date codes on some Snap-on tools - two (rarely) and even three (more rarely).
A careful look at the photos reveals that in this case, the “3” and the “4”
are on the separate legs. Multi-component tools like pliers it’s plausible to assume that the individual components were stamped prior to assembly, so late December and early January forgings could have been mated, producing something like that.
If both codes were on a single leg, then I could still see one shift was stamping a pile of them prior to assembly (the “3” is on the inside of the grip) late December, then January another shift doesn’t notice the stamp on the underside and stamps the cheek (“4”).
But multi-date codes also show up on individual things like sockets, extensions and drive tools. My impression is these are usually early production. Some people say this indicates they were stamped at time of production and again at time of sale, to track more specifically when a warranty would expire (S/O’s warranty evolved over time and varied with some tools). But this seems to suggest that not just factory, but also sales agents, had sets of the special font dies, which seems less manageable to me, and places a lot of disgression into the hands of sales. If date code stamps were all lightly-impressed, I’d have an easier time believing it, but many stamps are quite deep, leading me to think the equipment for making them wasn’t just a hammer.
And maybe there isn’t a single explanation that covers all multi-struck codes. If you search the
Vintage Snap-on thread with “multiple date code” the topic is discussed occasionally.