Variants of a mold are catalogued separately, so each one has its own sets and minifigs. Check these too if something you expect is missing here.

• 3183a Plate Special 1 x 4 with Towball Socket, Long, 2 Slots this part is a pair of it
• 3183b Plate Special 1 x 4 with Towball Socket, Long, 4 Slots this part is a pair of it
• 3183c Plate Special 1 x 4 with Towball Socket, Short, 4 Slots this part is a pair of it