According to some arrow charts I have, micro-flites came on the market in 1961 in yellow and gray colors. Two additional sizes were added in 1971. Depending on whether they are pre or post '71 and on whether they are target or hunting shafts, the spine for no. 9s ranges from 50-70@28. Most likely is 55-60#@28. Not uncommon for the yellow to darken over time. Very nice shafts. Were quite popular in the 60s and early 70s. Don't have much collector value as far as i know. Shoot em.