I have a MSD 6A analog box from the 90's. I also have a MSD start retard box plugged into it.

I lose about 4* with RPM. It started when I plugged in the start retard box.

I too have a collar on the dizzy shaft, but that made no difference.

It's my understanding the retard is normal. I'm OK with that.

That's all I know and have nothing of value to add.