Since this fitting is a straight thread, not a pipe thread, how is it
sealed.  Is thread compound all that's needed, do I need a copper
washer, or something else?

I believe the original fitting just had a crush washer of some type. I can't recall if I used a similar washer, or a "Stat-O-Seal".  I
think it was Fred who turned me on to these babies, and I use them
whenever I can.   http://www.gen-aircraft-hardware.com/stat-o-seal.asp
 
Cheers,
Rusty  <http://www.bakerprecision.com/statseal.htm>