1980 Mercury 7.5 not pumping water

Motor was working well last season but the flush attachment was blocked so I had been flushing in a bucket, muffs do not work on this model. During the winter service I unblocked the flush screw by drilling out the old one. Then i attached a hose and ran water through the engine, after a while i noticed less water coming out the pee hole, after investigation I realized gunk was being flushed out and had clogged the hole. I continued flushing and ran the motor for a while with the flush in and in a barrel. When I removed the flush the motor wouldn't pump. After investigation the flush had also clogged the impeller and it was slipping. I changed the impeller, tested the pump with an electric drill then assembled and ran the motor in a barrel and no water came out the pee hole. All this leads me to ask is the hole in the back of the flush hole my mistake when drilling out the old screw or intentional?

I have a 1982 model 9.8hp, which I think is the exact same platform. I will pull out my flush screw today and let you know what mine looks like.

What kind of gunk is coming out?

I looked at mine earlier and there is no hole there like your pic shows. Its just a smooth transition straight up into the motor.