UPDATE
There comes a times when you have to laugh at yourself for being so stupid,
as I will show in a moment.
The Manfrotto 679b monopod, for which I have been asking for help, comes with a "built-in" 1/4" - 3/8" and the brass 3/8" screw is, get this,
retractable !!
It is meant to just slide down, with an obvious spring loaded mechanism, to allow the 1/4" screw to be used.
So, here I am, trying to forcefully unscrew the "adapter" which, I now realize, is not an adapter at all but a built-in retractable screw.
Talk about stupid!
I messed up the outer (3/8") screw badly but can still use the "inside" 1/4" one.
I want to say this to myself: next time "read the manual" ... on the other hand, there wasn't a manual with this monopod and the seller didn't mention the retractable thingy, nor did it ever occurred to me that this was the case.
So, to make a long story short: my apologies to those who offered help. I will probaly order a new monopod if I need to use the 3/8" screw ... which is now totally banged up.
Cheers to all.
JP