I suspect the "PA" refers to the Spanish acronym meaning "A.P." or Artist Proof.

http://www.artbackers.com/product-category/archives/
Artist Edition Prints are unique pieces of art, and have been hand touched ord embellished (Retouchè) by the artist. These Editions using a differents tecnique of realization, and every prints is unique and different about other. These come in the form of A.P. (Artist Proof ) or H.P.M. (Hand Painted Multiple)

Artist Proof or A/P
Common practice is that 10 to 15 percent of an edition is reserved for the artist. In addition to the regular numbered edition, the artist usually selects a specified number of inventory for either his or her own uses, for a museum, or as the artist chooses. These proofs may be designated as artist’s proofs (AP, or EP in French and PA in Spanish) Print marking example; A/P 1/100 is the first print of an edition of 100 Artist Proof impressions. By art market standards Artist Proofs usually rank higher in value to S/N’s of an edition. All prints are accompanied by a certificate of authenticity.

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Note added at 36 mins (2018-03-01 16:14:36 GMT)
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http://www.simplyjackvettriano.com/blog/what-is-the-differen...
Artist's Proofs are exactly the same in terms of how they are printed, the colours, quality of printing, paper etc. Artists number them, often with Roman numerals and with the mark A/P. Artists sign them too, exactly the same as with the limited editions.

There are not as many artists proofs released however. Normally only 10-15% of the limited edition run is released as Artist's Proofs. The limited edition of Vettriano's The Drifter had a run of 275 and the Artist's Proofs released were only 30.