David.
I cannot quote you chapter and verse on these labels but they were the fore-runners of the postal charges calculator labels and the machines were made by N.C.R. I believe Richard Peck has written about the later ones in the ACCC of NSW Bulletin circa 1990.
I have seen them used for payment of postage and my example below shows use to acknowlege payment of the tax charge for unpaid postage.

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