I found this image on the 'net. It seems that what you have might be a hatchet shape Soldering Copper.
These are sold in pairs (one in use while other is being heated), the weight is for a pair, so your one being stamped 3 possibly means it weighs 1 1/2 lb. (a pair would be 3 lbs).
They can be used to do long seams on e.g. copper sheet.
They can be heated nowadays with a flame, blowtorch, but originally in a brazier or Blacksmiths furnace before the advent of electricity or gas flame. That might give you some idea of it's era, + your example looks 'homemade', probably by a Blacksmith, not mass-produced.
You can get more info/images if you do a google search using the term 'soldering coppers'. (I found lots of images but wasn't able to transfer them to here). Mc Master-Carr and others still sell 'soldering coppers'.
I'm thinking that your soldering copper has a curved 'blade edge' to fit the workpiece, perhaps solder-sealing a cylinder/barrel/fluid container?
T~