There's a 1 inch gap between the finished floor and the bottom of the drywall (which you found when you removed the baseboard, and it's along an outside wall?. And UNDERNEATH that gap, inside the wall, there's a drop of about 4-6 inches of empty space. Is that right?
The outside wall part does stump me a bit, because you don't usually find walls without sill plates along the outside perimeter. However, it is not uncommon in houses of that era to have interior walls built without sill plates (FYI, sill plates are a piece of lumber that's layed flat and the wall studs are nailed onto it). Eighty years ago, when your house was built, they may have just placed the upright wall studs directly on top of the subfloor and toenailed them onto that.
I would guess that what you may have is a void down into the area underneath the subfloor, and yes, it can get mighty chilly and drafty under there.
You probably can foam that 1" gap, but it will get pretty messy. What you might try doing instead is getting something called backer rod (which is like a 1" or 1 1/2" styrofoam rod) and smoosh it into the gap between the floor and the bottom of the drywall. Then replace your base board. It should fill that gap nicely without the mess.
If you haven't already done this, get yourself a stick of burning incense, and hold it in front of all your windows (be careful of those curtains!), and the baseboards, switch plates, outlet covers, light fixture bases, and register covers (with the fan off, of course), and the smoke direction will very quickly tell you where the cold air leaks are.
Where you are probably getting even more air leakage is from behind your wall switches and outlets. There are foam covers that go behind the switch covers that will help you eliminate those draft, too.
Another place that leaks like a sieve in older homes is around the window trim because the older windows have weight pockets on each side of them that are basically large shafts filled with cold air. I have found that making sure there's a good bead of paintable caulk all the way around the window between the wall and the window is very helpful.