Unless you absolutely have to remove them, don't! Most saw repairs (apart from blade straightening) can be done without disturbing this joint, so unless it's loose and needs tightening, leave well alone.
If you have a brace, try looking on Ebay for screwdriver bits - they come up quite frequently, and often with a 'double prong' bit specially for this job. You may have to slightly reshape it to fit your particular saw-nuts as they all seem to different, but make sure you get a good full-width fit or you'll chew up the nut even more than it already is. The brace bits respond well to a fine file, but use a fair bit of pressure and file slowly.
Alternatively, you could find an old, wide-bladed screwdriver and modify it as Jacob suggested. Again, make sure it's full-width.
If the sawnut slots are really badly chewed, find a small piece of hard steel (old screwdriver, perhaps?) and make a tiny chisel about the width of the existing slot with which to trim it up and maybe deepen it a bit. Clamp the handle down to the bench, go gently with the chisel and a small hammer, and the brass will cut easily - it's quite soft.
Just a thought about the age of the saws - split nuts rather went out of fashion around WW1 in favour of tubular nuts with full cross-slots - easier for a standard screwdriver to deal with - so they may be a bit older than 1940s.
