Alabama started using validation decals for the 1978 expiration year.  In 78 and 79 only, they issued different color decals for passenger and truck plates.  They also issued a third color decal these two years but I've yet to determine their use.  They also went to the added trouble to print "Sample" stickers, one would assume to prevent people from buying sample plates and robbing the sticker to keep from having to pay the registration fees.

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