Don't let "planer gage" make you think that this tool is only used on planers and shapers. I use my planer gages a lot! It's kind of a one size fits all set of adjustable parallels. It's a square, and most of them have a level. This is one handy tool. I use it for measuring dovetails in positioning tables I manufacture. Trying to use adjustable parallels is awkward compared to the increased width of a planer gage. Measuring dovetails is awkward at best since they get measured with precision balls. So you try to ascertain the distance between two balls, and you have to measure it on the crest of the ball. Nothing easier than sticking a planer gage in there and measuring it's faces.
The cylindrical tubes, normally a couple of them increase the dimensions that the gage can be used for. Measuring slots less than a 1/2 inch wide can be done with a multitude of tools, but the planer gage is just one of the handier ways to do it. Stick the tool in the slot, push the planer gage apart until it's wedged in the slot, and just measure across the planer gage with an outside micrometer. Any inside measurement with straight walls is quickly done with a planer gage.
And Of course they can still be used for setting toolheights on Planers and shapers, in case you have one of those out in your garage. I have four or five planer gages. They're available for free because people think they are antiquated. Fine by me- I just buy them for nothing. They are very finely crafted tools.