Especially with earliest cars, changes often did not coincide exactly with the model or calendar year, so it's not surprising to find, for example, a '79 with '78 characteristics. To further complicate things, these cars are so easy to work on and parts are so interchangeable, that over the years owners would mix and match freely. So if you find a car that doesn't seem to fit into any of these categories, don't worry about it. There's no premium for concours-style originality (although if you do luck into a true time capsule, please do try to keep it that way); you should be pay more attention to how well the previous owners cared for the car, how respectfully any mods have been done, and the presence or absence of rust.
X1/9 Mechanical components are quite cheap, but body repair work is expensive. It is best to buy the best car you can afford, paying particular attention to the body. Cars with any structural rust should be avoided. --Dom. 15:45, 17 August 2009 (UTC)
Background - North America
This section is pretty heavily biased towards the North American models, because that's what I know about. It would be a good thing if someone familiar with the European models were to put something in the section below.
The US-spec X1/9 came in three basic flavors:
1974-78 have a carbed 1300cc engine, a four-speed transmission, the shallow shelf-style dash, and a low engine cover that does not extend the width of the car. The '74 cars are also distinguished by a mechanical choke, small and attractive wraparound bumpers, an exhaust system that bolts onto the engine and transmission, different steering rack internals, an oil pressure gauge in the dash, and some other small differences. People sometimes install '74 bumpers on later cars, so these are getting a bit hard to find.
In 1979 Fiat added a five-speed transmission and a 1500 c engine with appreciably more torque. The "lollipop" flexible shift linkage was replaced with a metal linkage, the modern dash style was introduced, and the higher engine cover now runs the width of the car. The carb on these cars is the anemic and heavily emission-controlled 28/30 DHTA, and replacing it with the 32 DATRA from the earlier cars is a popular mod.
1981 replaced the carb with Bosch fuel injection and brought the lollipop-style shift linkage back. Fuel injection improves drivability and cold-start behavior, although the US cars are somewhat handicapped by a low-performing emission-friendly cam.