Hi I hope this info helps in solving your mystery.
Attached is a pic of 5 1969 blackless and backless proofs (against a white background to show color).
Although the backs were pinkish in 1969 it looks like it wasn't related to the cardstock as my proofs have a similar color as your pics.
In my opinion you have finished front proofs or unprinted blankbacks. Anybody out there have any pink blankbacks (pink colored but no print)? I haven't sent any...have seen some very light, almost white/tan backs with print in circulation.
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