Elucidate for us folks out in the hustings. What "burnings"?

I thought there was some recent discussion of this to give context. Maybe not.

The Heinleins (same XY, varying XXs) weeded and burned his personal and working papers at least three times - around 1947 (new marriage), 1967 (not certain why - but the great mass of material was either destroyed or passed to UCSC), further accession to UCSC around 1973-4, and a final burning when they moved from Bonny Doon to Carmel in 1986.