1973 model was the last year of SL350 series, they were all manufactured before the end of 1972 to avoid federal mandate of adding turn signals to all street legal motorcycles manufactured after January 1, 1973 (that's why all 1973 Triumph Hurricane was manufactured in 1972).
1972 and 1973 SL350K2 models (in gold or blue) are nearly identical, along with 1970 1/2 and 1971 SL350K1s (in orange, red or blue). The original 1970 SL350 (in gold, red or blue)was all different, along being 40lbs heavier and carrying CL350 engines.
Currently, I have three and I wouldn't pass on a 1970 model if one comes up in good condition (I did pass on one nearly a decade ago because it was missing too many parts). Finding original seat, mufflers and side covers are hard but it can be done. My own personal collection is nearly complete. I personally like the 19" front wheeled K1 model, I had one during my high school years too and they make a great cafe racer and also a semi-chopper. I made a photo review of that bike in my own ANGELS 750 #1 motorcycle comics series.
