If pressed, I will claim that I seldom buy individual braces. They accumulate via the medium of those package deals known as "the tool chest" or "the fiver for the lot" and so forth. Except it's not actually true. All too many braces of unusual size or design or make have followed me home simply because they were a brace of unusual size or design or make.
One such is this singularly unprepossessing 10" example.
Under the rust, there was a trade name revealed; Comet.
At least I assume it will be the same Comet; there might have been someone else using the name, of course. Anyway, I can't seem to find any dates for H & D Churchill outside the 1950s; but perhaps there simply aren't any. But in looking I did find this, which demonstrates that well-known chain of frustration from customer to retailer to wholesaler to manufacturer was alive and well in 1954.
I should pause here, and admit that every time I set forth to get clear in my mind which chuck type is called which, I slip and fall in a quagmire of patents and confusion, get a splitting headache, and run away before understanding anything. So you're welcome to tell me what this is called and I'll be entirely grateful and not in the least embarrassed at not knowing. My best guess was is would be classed as this one, but then folks mutter about subtle differences and... Gah. Whatever. It looks like this: