Gird your loins, It's out. Thanks to all of you It came out, not willingly but it did. There will be a picture posted soon. Mostly I have to say the penetrating oil seemed to be the most effective in all of this. I did a bit of several things before I clamped it in place and after the last treatment of penetrating oil had drained away. Then I gently tapped the best surviving section of original slot and it finally started to move. Ten minutes later the screw was in my hand. Wow, what a relief. So, do I try for a replacement screw(?) or revive this one. Seems like the best and most affordable alternative would be a used replacement?

Thanks,
Carl