Not to create hostility, but as a retired machinist and mechanic who sometimes used both types of plugs, I have to say there is somewhat of a distinct difference. The welch plug is like a flat coin that is pressed in the center to become slightly rounded, and when installing, the outer diameter edge is increased slightly by pressure applied on the center of the top of the convex, rounded surface. While it may look similar from the top of the sometimes-rounded surface, the traditional freeze plug has a side surface forming the working outer diameter, that is at approximately a 90 degree angle from the rounded surface. It looks as if you attached a bushing to the concave bottom of a welch plug; some of the old freeze plugs don't even have the 'domed' shape, but are flat across.
As we know, they both have a similar function, but there are places where a freeze plug would not work so well, such as when the hole to be plugged has a very shallow shoulder. I chose to go with the welch plug.
I found it interesting that most of the auto parts clerks and industrial supply people I spoke with had no idea what either one is. I guess my age is showing by way of my technology. LOL
