So over 80 years have passed since the L134 Willys 'Go-Devil' engine was first marketed in a vehicle, and in that time a number of original parts, original replacement parts, and after market parts have been available on the market. For most of us rebuilding a WW2 G503, we'll either find a 1940s wartime version for the MB/GPW, or we will find a 1946 and later pump. The early cj 2/3 etc L134 equipped engines all shared a number of parts, including the water pump which is interchangeable with the WW2 G503 version. You may ask if it's interchangeable, then who cares as long as it works? Fair question, for many people there is nothing wrong with using a later style water pump in their WW2 G503, but if you are trying to restore it to an accurate representation of a wartime factory delivered condition (or even a wartime field 'motor pool' level), you will need the proper style parts or it will quickly be pointed out as incorrect by anyone remotely familiar with the correct parts.
So just what was the difference? Good question! I see a lot of people who haven't seen enough of either version of the water pump to tell the difference. Now the part number is the first step in validating the right era part for your G503, but it's not the end of the line. You see often the part number barely if ever changed after the war when it comes to the MB version, while the GPW one seems like it would be so distinctive that it would be a shoe in, it isn't always since some early postwar copies have turned up using the GPW part numbers. The next and biggest step in the verification is visual, since the wartime G503 version looks a lot different from the postwar models. During WW2, the G503 left the factory without a cab heater (under the dash), but a kit was introduced during the war to allow vehicles to be retrofitted (if needed) with an under the dashboard heater. The heater had an electric fan to blow the air, but the heat was generated by the hot engine coolant passing through the heater. The water pump and the cylinder head of the engine had a special area cast to allow a hole to be drilled and threaded for a special fitting that would allow such a heater to be installed. If needed, they would drill a hole, thread it, and then hook up the heater. This meant that it would be harder to install in the field, since a drill (or drill press) and special threading taps would be needed, which many units did not have in their equipment.
After the war ended and civilian production of the G503 commenced as the cj-2a model, the heater was retained as an optional kit from the dealer, and at some point the water pump was redesigned to be drilled and threaded from the factory, eliminating the need for special equipment and procedures to modify the pump as the WW2 version required. The redesign added a second port to the water pump for a heater hose, if needed. Otherwise the two ports were capped off with threaded plugs until needed.
And a third feature that changed over the years is the pulley itself, with some being cast metal and some being stamped steel.
This little video I put together might help illustrate the differences in the main castings and the different heater ports found on later models. Today aftermarket and reproduction pumps are available (most commonly in the postwar version), but some companies (such as Joes' Motor Pool in the UK, and I know Ron Fitzpatrick Jeep Parts here in the US stocks them) do offer reproductions of the wartime G503 pumps in case you can't find an original to rebuild, or don't want to rely on a rebuilt pump but at the same time want something closer to the original wartime factory version.