The saga begins-- originally, the T/C plugs were deeply concave. They worked fine with loose powder, still do. But, customers were crushing pellets-- and complained.
So, Thompson introduced "Stage II"- primarily a solid plug, with a very small dish. The solved the problem of crushing pellets by customers who got a little too eager with hard-to seat sabots. This introduced a new problem-- customers complained of fouling build up 2" from there breech out. T/C is still not sure why / how, but they were able to duplicate it.
Finally, they redesigned the plug again-- same as the original dished plug, but with a heavier outer ring to provide support for pellets. Best of both worlds, the crud buildup eliminated. That is the "Stage III" plug.
The original plugs were / are perfectly fine for loose powder.