I went down to the barn and found the old TC that's lying there, along with the hex plug I put in when I robbed out the original (complete with the allen socket I booger-welded in to get it out). It is indeed very slightly tapered, and washerless, but it is not NPT, it is metric. The threads are finer than NPT and the taper very slight.
If you can get it out at all, one option here is to find a 3/8 inch drive socket (I always have a bucket of crappy asian ones handy for this) and weld it to the face of the plug. There's plenty of clearance for this, and you'll then have a nearly foolproof square socket to use next time. I've done this over the years with a number of boogered-up drain plugs, and as long as you have space, it works fine. The excavator in my avatar has that on its oil drain, in fact.