It doesn’t look like any piston ring expander I’ve ever seen, either. I meant that it looked to me that that was what its function was.
I follow. However, this…
…the two sets of jaws stay closed and spread apart, away from each other…You can also see that this last action involves a very small range of motion - i don’t think the distance between the two sets of jaws increases by more than about 1/3 of an inch, or about 8mm.
…isn’t nearly enough motion to expand a ring enough to install it.
I’ve got Mr Cranky on the case. He’s on an email list with a bunch of galoots who love old hand tools. They mostly know woodworking tools, but who knows what one of them might know.
Yes, you’re probably right. The 8 mm stretch only gives a 2.5 mm change in diameter.
When you release the pressure on the handgrips, everything happens exactly in the reverse sequence. First, the coiled springs pull the two closed sets of jaws back to their earlier position closer to, and parallel to one another. Then, as the handgrips are released all the way, both sets of jaws open together until the tool is in the rest position, shown in pictures tool1, rest, and rest closeup.
Then:
Yes, troub has it right. There is a spring that pushes the handles apart, but the springs on the device itself definitely do not look like add-ons - they serve to keep tension in the tool during the final, spreading stage, as well as to bring the two sets of closed jaws back to parallel.
Ahhh… with that last clarification it becomes clear what that tool is for. Here you go!
Way off topic, but…
I have to tell you that I browwed (snooped) your galleries a bit. You’re a fabulous photographer! Is it your profession?
Since the tips are not sharp enough to get a bite of something they probably fit into some grooves or slots in something. When you squeeze the handle they close into slots and spread. Could they have been used to remove the metal ribbons around the wheels from old “horse drawn buggies”. I’m sure those ribons were nailed on, but after removing the nails they probably still stuck to the wood. You probably would not need to spread the ribbon much…just enough to break a tight seal.
Before reading the link that astro is providing, my guess now is that it’s used for plumbing. A ring/clamp could be held open enough to fit it over a pipe, then released to be welded or screwed in place. (I’m assuming a clamp with screw holes that would line up on the outside so a screw could go through them to tighten it down onto a pipe.)
Am I right? Close?
Ooookaayyy…Well, I stand by my current suggestion.
They are “spring retaining ring” pliers.
Go here
http://www.milbar.com/tools-retaining-ring-pliers-fixed-tip-pliers-ergonomic.htm
The wallpaper shows a bunch of the rings. Newer versions of the tool can be found there.
The surfaces of the tool in the close-ups look kinda forged, as opposed to mass-produced finish. I’m not saying one-of-a-kind, rather maybe limited production. ???
My reaction to that has to be…no way on God’s green earth. The tool in question does not have projections to engage the holes in the snap rings, and its jaws at rest are WAY too far apart to do any good with those rings.
Some good guesses here, but I’m going to say its just a simple hand clamp for use on a variety of things.
I don’t think they are general purpose ring pliers, but for some specialty use like, for example, a 1930-37 Mercedes 770 brake assembly.
There are a gazillion types of retaining rings. Some have holes in the ends, some have bends, some have dowels, etc.
They clamp down, then spread apart, right? To what other use would those actions be put, than to spread a springy thing?
FWIW, this was my first guess too. The only thing that kinks this hypothesis is the “standard measure” imprinted on the side. This makes me think it was in indutrial type thingy-ma-bob used in the production/maintenece of something that had to conform to gov’t standards. My WAG, it relates to automobiles, trains, or something along those lines.
Since it doesn’t move very far, my guess is that is designed to seperate things with a good amount of force.(out of curiousity, when you are squeezing it and the jaws come together do the jaws squeeze really tight or does it pretty much meet and then start seperating).I’m guessing that the jaws don’t grip anything, but instead you just put them around some sort of bolt and the jaws just give extra surface area to push against. Kind of like a gear puller(or pusher would be more accurate) I’d guess it has a very specific purpose. But this is all a WAG.
Looks a great deal like the cable stretcher used for stretching a bicycle gear cable to put tension on the deraileur when setting the gears.
http://www.cambriabike.com/tools/park_bt_2_cable_stretcher.htm
Not really.
IMHO, of course 
Squint