What is this?

It’s a slug cutter

Corroborating the Good Buddy above, Slug, Cutter, and (lead) lead and rule cutter, (Never steel cutting rule)
Old but probably, with cast material base and probably good quality *Blades*, top and bottom, which can usually be re ground, still a good proposition.
Is the return spring in good condition,?? The original back stop/lay edge may have been lost, but that fitted quite serviceable..

Should find provision on the Top/cutting blade provision for adjustment after regrind and or wear, usually 2 securing screws and sometimes 2 adjuster screws to set the top blade for the right approach to the material, usually slightly Guillotine action rather than Parallel cut, slicing usually leaves a better edge on the cut, especially cutting Lino slugs or 12 point strip material.!!! . . Blades can be honed as with Chisels and Plane Irons. ***
***I.E. Little tiny gadget/tool normally for above blades, the blades are clipped in, the angle is set on the tool and merely honed down to a keen edge.

This lead and slug cutter was made by Kelsey over many years. They are simple, easy to use, but on repeat cuts at different times not terribly accurate. The gauge that is held in place with a thumb screw is not indexed except visually to the pica and half pica markings. A standard could be used for a setting but in my use of this cutter many years ago I never managed to get that sophisticated and instead found using someone’s printer’s saw was far easier and definitely more accurate. These cutters were inexpensive—around $7 in the 1950s—and for traditional letterpress using real type, a good supply of leads and slugs is required.