Radiator drain plug problem

Good day,

Went to drain a bit of the coolant out of the radiator from the lower valve, and it immediately started to leak. Felt like something was loose, then the bung came out along with the valve. Luckily I had enough containers handy to catch all of the coolant. I can't seem to find this bung anywhere in the shop manuals and suppliers, and I don't see how it supposed to fit in the radiator without falling out. It just slides in, and not particularly tight. The only thing I can think of is it's supposed to be brazed on. I guess I could just use JB Weld on it?

Thoughts?

The bottom of radiator should be brass so likely the boss was soldered on with a good fillet to strengthen joint
If not sure how to do it …
The high temperatures of brazing would melt the soldered fins into bottom,
Remove rad and lay flat
Clean up both parts with rotary wire brush in drill/ emery cloth.
Clean parts
Smear with flux
Tin each part separately with blowtorch with lead solder
Put the boss on rad heat up and feed in extra solder around outside to make fillet
Let cool naturally