Those are, in fact, "caliper gages".
Prior to the readily available micrometer, much measuring in the machine shop used plain calipers, inside and outside.
Much of the work involved 'fitting' one piece to another, so commonly, the more difficult piece would be made, then the other one fitted to it. A plain caliper, say inside, would be adjusted to the first piece, then the mating piece would be measured by an outside caliper referring back to the inside caliper.
But what to do when a particular size is required?
These standard 'caliper gages' functioned as master gages, similar to modern gage blocks.
Calipers were set to these gages, rather than some mating part, and absolute size was possible.
(Keep in mind that NOTHING is absolutely perfect, but a good part is within acceptable tolerance.)
They were relatively expensive in the day.
Contrary to much foolishness on the internet today, they were NOT used as plug gages to actually measure a workpiece, but only reserved to set calipers, just as gage blocks were originally intended.
The versatility and availability of gage block sets rendered these obsolete around WWI, and today asian gage blocks are so inexpensive they are commonly not treasured as they used to be.
