recently purchased a 91 F150 4WD, jamb tag say 19N, does any one know what the "N" means, I know they are 3.55

 

Haven't seen the N after the number. Usually posi has H19, the H meaning Trac Lok

 

I know that's why I was curious on why was 19N instead of in N19

Did they use a different coding system and 91

 

19 3:55 NLS Non Limited slip I'm thinking the N is for NON
H9 3:55 Limited Slip

 

Jack up the rear diff and spin the right wheel forward. If the left wheel goes forward it's limited slip. If the left wheel goes backwards it's non limited slip (open).
Driveshaft should go around 3 1/2 (3.55) times for the right wheel to go around once for 3:55 gears.
Mark the tire and pinion with chalk to make it easier to count.