Here is a quick article designed to de-mystify the VW "Type" designations. Read it and put your confusion to rest!
The VW Bus is a Type II. It is called that because it was introduced as the second type of vehicle VW made, the first being the Beetle (and why it is referred to as the "Type I"). The Bus used the same style engine as the Beetle up to and including the 1971 model. These were four-cylinder horizontally opposed ("flat-four") engines introduced in 1948 at 1100cc and 25hp, and evolving to 1600cc and about 58hp in 1971. While it is true that if one were to disassemble any of these engines down to the "long block" (valve cover to valve cover, flywheel to pulley—i.e. no cooling, intake, or exhaust systems), the Type I (Beetle/Karmann Ghia), Type II (Bus up to 1971), and Type III (Square-back, Fast-back, and Notch-back) they all use the same basic engine. However, as suggested above, the cooling, intake, and exhaust systems all vary a bit between the models. For example, the Beetle uses twin tail pipes and corresponding muffler, while the Bus uses a single exhaust pipe and corresponding muffler. The Type III employs a rather flat "pancake" cooling system design, while the Bus and Beetle use an up-right fan shroud system. So when VW parts people, books, or parts catalogues refer to an engine as Type I, II, or III, they are referring to the entire engine assembly, not just the long block.
In 1972, the Bus switched to the "Type IV" engine. It is referred to as a Type IV because it was originally designed in 1968 for—and fitted to—the VW 411/412, the fourth type of vehicle VW made and thus referred to as the "Type IV." This too was a horizontally opposed four cylinder engine, just larger inside and out. This basic engine was used not only in the 411/412s and Bus (from 1972-on), but also in all air-cooled Vanagons from 1980-1983, and in the Porsche 914/4. That is why many people incorrectly refer to the later Buses and Vanagons powered by the Type IV engine as "Porsche powered." It was, in fact, the other way around: the Porsche 914 was "VW powered." Unlike the Type I engine, this engine was never made with an up-right fan shroud, but rather only in the flat pancake design similar to the one used on the Type III. The first Type IV engines up to and including 1974 were 1.7 liter (69mm stroke/90mm bore). The 411/412 and 914/4 were electronically fuel injected, while the Bus had twin carburetors. In 1975, the displacement increased to 1.8 liter (same stroke as the 1.7 liter, but larger 93mm piston), and carburetion was dropped for electronic fuel injection across the board. In 1976, the engine size was increased again to 2.0 liters (71mm stroke, 94mm bore). The only other significant change took place in 1978 with the introduction of hydraulically adjusted valves via hydraulic cam followers instead of solid ones.
So, when ordering parts it is only necessary to know the year and model of the vehicle. For example a 1976 Type II. However, it gets tricky getting the right part for vehicles that have been modified, or have had engine swaps. In that case it would be necessary to know what year and model from which the engine was removed, assuming of course nothing was changed on the cooling system, exhaust system, intake system..... you get the picture.