is because some of the aftermarket pumps are designed different from OEM. On the one you have the bearing cartridge slid out the front allowing the rotor to get into the housing. Had one do that to me to. The OEM style pumps have a lip cast into the front of the housing under the fan mounting hub. This prevents the guts of the pump from coming out the front. It's probably the fan that pulls it out.
I like the belt idea. It would definitely be nice to have more wrap on the alt belt.