David Weaver
Most of the marples chisels retain a flat bevel. A couple of sloppily done roundovers on the top edge.

All of the IH sorby chisels are rounded except those used enough to move the bevel far back (some of those continued to be rounded)

You'll have to describe the mechanism that would cause users who don't know each other to only do this to IH sorby chisels. It's remarkable.

You have an argument with a blogger so the extremely unlikely becomes likely.

All of these things point to a conspiracy against someone who looks at tool catalogues in 1973 (none were IH sorby), had one of these in hand (not IH sorby), and refuses to believe that anything could've been made more than one way.

As you find that you're less and less likely to be correct, you point to double irons and a chisel from a century before.