There are 2 types of fuel tank bodies. 

#1 - M151, M151A1 & M151A2 Pre emissions system tank & #2 - M151A2 emissions control tank.

There are 3 types of complete fuel tank assemblies.

The M151 & M151A1 came from the factory with a electric fuel pump in the fuel tank. This is the 8" bolt in "insert" with 3 connections on the top. 1 - Fuel pump power, 2 - Fuel out of tank to carburetor, 3 - Vent to air cleaner. Separate from this is the Fuel level sending unit.

M151A2 Pre emissions used the same tank as above, but had a mechanical fuel pump, and the 8" bolt in insert 3 connection are as follows. 1 - Supply to fuel pump, 2 - Return for excess fuel from carburetor, 3 - Vent to air cleaner. Separate from this is the fuel level sending unit.

M151A2 Emission Control Fuel Tank, 9 different holes on the top of various sizes from 1/4 inch to 3 inches. A mess of steel tubes on top for the emission control. Fuel out & return and fuel level sending unit.

Pictures below will help sort out what this all encompassed and looked like. Also a detail of the fuel caps correct and usable on your vehicle.