If you look at the outer edge of the head, and it is 4 round bars, it is 72 or earlier, then if it has a seperate cable to each carb it is 69 or 70.
If it has 3 sorta oval bars, then it is 73 or newer.
Then if it has a shift pattern cast into the transmission cover it is 75 or newer.
77 & 78 K have studs on the exhaust ports, and will have an output shaft ( for front sprocket) with no groove in it for the reatainer.. and should have a wider sprocket (630 chain if stock)
If you cant narrow it too 2 years you are doing something wrong..