Yep, definitely a bottle capper. I've got a couple of them. During prohibition many otherwise-law-abiding citizens made their own beer at home. My great-grandfather made it regularly in his bathtub, according to my gr-grandmother. She hated having the tub out of commission
...(and having to clean one's self in some other fashion). 
Anyway,
lots of folks made home brew, and bottle cappers were hot items at the hardware stores. Also, some folks made their own root beer or ginger beer.
These days many folks keep them around for use as nutcrackers...a job which they do REALLY well. You just place the nut on a brick or block of wood under the moving ram, pull down the handle, and voila'. The leverage you have this way gives you
much better control, so you can break the shell without crushing the nutmeat inside...unlike a clumsy hammer or those useless V-shaped nutcrackers which come in holiday gift packs.