That definitely looks like a threaded fitting.  Maybe some penetrating oil or spray will help to loosen it up.  Usually it's just a matter of re-packing the valve assembly with the specified type of grease or whatever once you get it apart. 

I don't know what to tell you about the limited access.  Depending on the angle you have available, you might be able to use a basin wrench on it (see link).  There's currently an ad on SF Bay Area CL for various old stove parts.  Maybe the seller of those would have some information to share.  Otherwise, your best bet for advice might be to contact the remaining repair shops in L.A. that specialize in old stoves.  I think one of them is in the Burbank/Glendale/North Hollywood area.