Definitely soldered!! You'll need to clean the area beyond clean solid core solder and a lot of flux and a good torch to float the solder. Don't get the core too hot, or you'll lose the soldered core seals. Make sure you check for leaks in a tank of water with a little air pressure in the heater core itself. I've seen them braised on in later years, but even my 1958 Chevrolet heater core bracket was soldered. Hope this helps.
Tim
