differences in injectors?

['89GTStang]

I have been having problems getting my mustang to idle after replacing the intake and pulling the codes it shows a 91 which says "o2 sensor problem, faulty regulator, or out of balance injectors" the o2 sensors have been replaced, plan on trying the regulator as soon as I figure out my current question which is about the injectors. It had the stock injectors and I happened to run across a set of relatively new 19 lb injectors for $20 so I coulden't pass them up. I put them on an the car woulden't even attempt to fire. I put two of the old injectors back on. (just two because that is all I could get to without taking the intake back off) and it started firing of those two cylinders. My question is were there some differences between the injectors of different years or something? the injectors look identical with the same color tops so they should be 19 lb but I don't know why they would not be pulsing? any ideas?