Rear Caliper??????
 

I have a 1990 Buick Regal Presidential I need to replace the rear calipers but the choices seem limitless I just bought the car from an estate and the original owner has passed away and paperwork is a little thin. The choices at Rock Auto are sports suspension, emergency stop and except emergency stop. I can find no numbers on these calipers to give me a hint and the pictures look all the same. It has the pump to engage the emergency brake. Part of the reason for the replacement it will not stay locked in and the piston does not hold. How do I tell what I have. They need to be ordered so I can not match them up first.

Thank you