J. is correct. For side valves. It would go between tappets so you could adjust the valve with one wrench in one hand and tighten the check nut with a wrench in the other. That plunger is to lock it in place. Prior to helpers or third hands like this mechanics would make shop-made tools just like this, which is undoubtedly where Snap-on got the idea, or do the sleight of three wrenches in two hands trick. Examples and catalog pages were posted on the 'Tappet wrenches' thread, as well as a page from a very early 20's industrial trade mag in which instructions for making one in your shop were provided in an article.