There are two alternators, a roundback and a squareback.
Single or double pulley is for AC and double belt systems like motorhomes or trucks with big fans.
You have and want a squareback, but with a single pulley, but last I got one they told me they only had 2 pulley because it would work for a car or 2 belt system.
(Probably BS, but it was all they had around here locally)