I assume you inherited the pool and that someone has worked on the pump before. Motors of that size generally have 4 long thin "bolts" that hold the front and rear plates together. Each plate has a bearing for the armature shaft. My pool pump guy said that it is very difficult to remove them without breaking. He applied a lot of heat to the front plate at each bolt location while trying to remove the bolt. Eventually they all broke loose. I suspect the bolts are broken off in you motor. I'd be concerned if the plates are only held together by friction, although the motor has apparently run this way for some time. But the plates/bearings determine how "true"the shaft will turn. If it's not right and wobbles a little it'll eventually wear the seal again. I assume you close for winter. That would be a good time to check it out. You may need another seal if you take it apart. A motor or pump repair shop could probably get you the bolts

Not sure about the impeller screw. I watched the video that Mark posted. Looks like a "safety". Presumably the motor spins in a direction that will tend to tighten the impeller on the shaft. It may be worth getting one if you plan to take it apart again. Looks like it's reverse threaded and may only be available from the manufacturer. There are websites that supply OEM parts.

Interesting that the video recommends lubricant for assembly. There are also restrictions on lubricants for o rings on devices like a chlorine feeders.

www.ereplacementparts.com has a diagram, parts list and prices for your pump. I Googled "haywardsp3400vsp parts". You might want to replace the "impeller ring" while you're at it. This is a wear surface between the impeller and the diffuser.