✅ SOLVED Hand sized file device.

[FireFlowerPower]

Its 9 1/2cm by 4 1/2cm, to just perfectly fit in your hand and vaguely resembles a brass knuckle design tho its clear thats just a coincidence.
It has a file in the center of it that shows some use. Most likely something narrow was placed down the center slit to file it down. But other than a blade I cant image what else would fit, and why would you want to dull a blade?
The only markings are an E on one side and a C in the same location but on the flip side. I suspect its part of a larger device, at the very least to a brace of some sort.
As you can see by the images there is not much to it.

 

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[nhbenz]

An ice skate sharpener I think

 

[DBANDGB]

i have one from my grandfather. before you sharpen a hand saw, you run this over all the teeth to make them level. then you sharpen the teeth with the small files.

 

[vhs07]

This one is described as a "

Vintage crosscut saw aluminum jointer, file, raker gauge tool"
​

 

[nhbenz]

A versatile little setup! So many uses you'll wonder how you got by without it.
We may have to find one with the original box to be conclusive. This seller thought it was for skates: ANTIQUE ICE SKATE SHARPENER COMPLETE

 

[nhbenz]

Actually I now see that DBANDGB and vhs07 are correct, as many examples can be found just calling it a saw jointer, and many compare this model (often unmarked) to the Stearns model.