Union membership

Is there any reason why union membership is only available in ipums cps from 1984: I think BLS starting asking this question from 1973. It would be great to have the data going back further. Thanks.

IPUMS CPS offers unharmonized variables of union membership for all basic monthly data in 1983 and the 1971 ASEC. I spot-checked MORG files from NBER, reviewed UNICON documentation, and looked at ASEC codebooks and am not seeing evidence that these data are available. However, if you have more information on where you suspect union data are available in CPS, please let me know.