Original tap? It's just a solid body with a tapered hole where the tapered plug fits. On the bottom is a shaped washer, spring, and cotter pin. Remove the cotter pin, and collect the spring and washers. The tap will probably fall out, but you might have to turn it a few times. The spring tends to rust and get weak, or even break altogether. I found a suitable substitute in the parts bin and cut it to length. Sorry, no idea where it came from.
In a pinch, I've been known to wrap a worm-drive hose clamp around the handle and the body, so it forces the plug into the hole. I was on a drive away from home when I did that; then forgot all about it and drove that way for several years. Discovered it when I went to change the coolant the next time
Note that the radiator tap and the block tap are actually slightly different even though they look the same. The block tap has straight threads and takes a fiber washer, the radiator tap has tapered threads.
Also, there is a standard radiator drain valve that will fit and work fine. Doesn't look original of course, but might be an alternative. I always take the original tap out when taking the radiator to a shop, because they will replace it.