You have a two-groove pulley on an 18GB motor? That's incorrect. You should only have a one-groove pulley. The pulley only had one belt. It went from the crank, water pump, and generator. There is no second belt,
But, to keep things simple, probably best just to order the later water pump. The one they sent you is for the GB motor. Your "original" was not original.
It is not original. You have a '72 B. I assumed it to be in the 64-67 range because of the GB motor. Somebody put an early engine into your MkII MGB.
That's actually better in my opinion. The GB motor is considered superior to the later ones by most folks.
I'm guessing that you have quite a few mods on that car. Different back and front plates as well to adapt to the later all-synchro trans, later starter, etc. etc. They adapted an early engine to fit your '72.
Brian Maesk