ASEC Family Weight in 1976

I am trying to find the data for family weight in the year 1976. I am using Unicon data for 1962-1997, however the variable famwgt is not available for 1976. I see that IPUMs ASEC microdata has data for family weight in 1976, where is that data coming from?

IPUMS CPS constructed the 1976 family weights using the ASECWT value of the first person in every family (as defined by FAMID). This was found to be equivalent to other years in which ASECFWT had a simple source variable in the data.