My service bulletin #15 must be different than yours because I don't see the diagram that you referring to. It is dated 1/31/75 but refers to the 3/16/70 date. I have attached a copy of mine that you can compare. It looks like it's a revision to the original service bulletin.
yes, there is also the 3/16/75 sb revision which deals only with the 28 parts. the original #15 has sketches/measurements of springs, slides, cables, tops and caps. also info, sketches, on the rr.hub cushions, frame modifications for thr.cable routing, labor rates, dealer stock, replacement parts and shipping the old parts to the easter bunny.