That's quite a nice old mortise chisel, and I believe the handle is original. A little work with 0000 steel wool should let you see the maker's name. I'd find it hard to pass one of these up.
I wouldn't worry about messing up the rust. There are a number of products you can use, like Evaporust and Naval Jelly, but that same steel wool with some penetrating oil or WD-40 might just do the job.