The only important holes there are the two larger round holes, they are your water intake and exit. The small ones do nothing and I’m not sure what they are for. Some nos gaskets have a hole, some don’t. The larger oval shapes are just part of the casting shape.
The lever on the back was an option when new, some have it some don’t. It is a factory item.
Here are pics of my KD-15 top of tower and bottom of powerhead. You can see in the first one the gasket covers most of those cavities. Still not sure why one small hole is open, it Seems to be a drain but as you said has no corresponding hole on the other part.
