Distinguishing 1970-1973 fork underbrackets can be tricky. I've mis-identified them in the past. 1972 and 1973 are one-year-only, and are particularly hard to differentiate. eBay sellers often list them incorrectly. It's easy to tell 70-71 lower triples from 72-73, because 1972 and 1973 marked a change to disc front brakes, and they added two threaded holes to the lower triple to hold a brake hose stay (green arrows). That area was completely smooth on the early 70-71 triples. Later 70-71 triples had two cast-in, but un-drilled bungs (red arrows). I have tried to figure out what those castings are for. I thought that maybe Yamaha was re-using a lower triple from a different model, but that does not seem to be the case. My best guess is that Yamaha was anticipating the change to disc brakes, and that the original plan was to have the brake hose stay bolt on from the side, rather than the bottom, which is what they ultimately went with. There are more differences between the early 70-71 steering stem and the later 70-71 ones, but they are completely interchangeable. In general, the early lower triple was a bit on the weak side, with a 27mm stem. This was changed to 30mm, which remained the diameter throughout the rest of production, all the way up to 1983. In order to accommodate the lower bearing race, a sleeve was added to the 27mm stem. The bearing races are identical. The steering lock socket is an added, welded piece on the early stems. The steering damper mating surface was also a separate, welded-on piece on the early stems. Changed to a cast-in surface on later 70-71 stems. Another view of the steering damper differences. Onward to 1972 and 1973 differences. This is the tricky one. The only way to reliably tell a 1972 yoke from a 1973 yoke is to measure the distance between the fork tube holes. The 1972 spacing is 175mm, the same as 1970-1971. In 1973, that spacing was increased to 185mm, and that spacing remained for the rest of the run, up through 1983. The 1970 and 1971 lower triples used a single 10mm pinch bolt on each side. 1972 and 1973 used a single 12mm pinch bolt on each leg. 1974 and all later models used a pair of 8mm pinch bolts on each side.