On Fri, 06 Feb 2004 18:52:39 -0600, wrote:

I once saw a plumber remove a similar plug that had been in place at
least 25 years. He used a sharp chisel and 4-lb hammer, the chisel
digging into the plug at an angle so he could apply tangential shock
with the hammer blows. After about 6 hits he could unscrew the plug
the rest of the way with channel locks.