Spicer series ujoint have outside outside clips that hold the joint in places.
they 1210 thougth 1550 measuring cap diameter and outside of cap to outside of cap will determine the serie that you have. your end yoke on your transmission and differential will have tabs that hold the ujoint from moving side to side.
Saginaw (GM) and Detriot (Dodge) :
This series Usually have inside clips, measuring Cap diameter, and inside end yokes on transmission and differentail will determine which size is needed. No Tab on end yokes, inside clips on the ujoint, keep the ujoint from moving side to side.
Notice Left end yoke has Tabs that ujoint cap butts against, the Right end yoke does not it uses inside clips that snap on to the Cap. This should help you determine the different types.