I was sorting through some old papers tonight, including old tax returns. I decided I didn’t need to save the instruction booklets, but I’m glad I didn’t just recycle them. Until the 1994 tax year, the Feds printed our Social Security Numbers right on the mailing label; starting with that year, they moved the peel-off label (complete with SSNs) inside the booklet, at least through 1996, which is the last booklet I found. The State of California printed the numbers on the mailing label at least through 1997.
I bet they don’t do that today.