If you walked into the local Kawasaki dealer say in the year 1990 and asked for a 250cc motocross bike, they would have shown you the KX-250. KX is the model series code for their motocross family of motorcycles, and 250 designates the engine displacement as 250cc. When Kawasaki switched to all four stroke production, they added the letter "F" after the letters "KX" to designate the change. If you were to step back in time to the year 1969 and ask the same question to the Kawasaki dealer, he would have shown you an F21M. Up until around 1974-75, kawasaki used a complete different model code for their motorcycles. They weren't alone as Honda, Yamaha, and Suzuki all used a different system in their earlier production models. Honda moved to the present code system in the late 1960's, Yamaha made the switch beginning in 1974, and Suzuki sometime around 1967. Also worth noting, is that when the code system changed, some models were dropped from the line, rather than recoded in the new system.
So, someone gives you a hot lead on a bike in the back of an old barn, which you can get for a song. Naturally, you jump at the oppertunity even though your spouse may not be so thrilled with your diamond in the rough. You get it home and start writing down the serial numbers, and the data on the data label if still present. Your bike is a Yamaha two stroke enduro, and the engine is a 360cc.....your friend, whom gave you the lead, said the bike is a DT-360, but the frame says RT-1, RT-2, or RT-3, and you are suddenly very confused. Since i know the Yamaha codes the best, I'll attempt to clarify how this evolutionary change works.
In 1968, Yamaha manufactured the first, purpose built dual purpose motorcycle, to come out of Japan. This motorcycle was a 250cc piston port two stroke dirt bike, with enough lighting and instrumentation to make it street legal. When I say purpose built it's because earlier "dual purpose" offerings, then known as "street scramblers", were little more than street bikes with their exhaust pipes run higher, and possibly a bash plate installed, and were next to useless off road. The new model was given the model code DT-1. As the bike evolved the code was changed to DT-2, and with more changes DT-3. Then starting in 1974, and with more changes, the code was changed to DT-250-A, the "A" meaning the first year of production of that model. The 1975 model would have been DT-250-B, and so forth until production ended. The following list hopefully will clarify all of this.
AT-1, AT-2, AT-3 = DT-125
AT-1MX (stripped down at-1 enduro with GYT kit installed) = stand alone model
AT-2MX = MX-125
CT-1, CT-2, CT-3 = DT-175
DT-1, DT-2, DT-3 = DT-250
DT-1MX (stripped down dt-1 enduro with GYT kit installed) = stand alone model
DT-2MX = MX-250
RT-1, RT-2, RT-3 = DT-360
RT-1MX (stripped down rt-1 with GYT kit installed) = stand alone model
RT-2MX = MX-360
Please keep in mind that the above listed models were constantly evolving from year to year, so when I use the = sign it doesn't literally mean the bikes were identical, but rather to trace the lineage of the model over the years. Also worth noting is that Yamahas' intent with the DT-1 was to produce a motorcycle that could be kept stock as a dual purpose, or modified for motocross, desert, or enduro races. They even offered a factory gyt kit, which stood for genuine yamaha tuning kit, to convert a stock dt into an mx bike. As time rolled along and things evolved, it became apparent that seperate, purpose built models were needed rather that expecting to do it all with one bike. By the mid 1970's the consumer had three models to choose from, the DT-250 enduro, the MX-250, and the YZ-250. The MX version could be used as a play bike or a racer, while the similar YZ model was specifically built for racing. Eventually, the MX model was dropped and a new model was developed for cross country competition, the IT-250. Evolution is a funny thing..kind of like watching a child grow into a man, they look very different as children as opposed to a grown man but yet they are the same, just matured. The below pics illustrate the evolution of the 360cc Yamaha.