Axle code

[custom63]

Hey everyone.
Im looking to pick up a replacement rearend to upgrade my rear axles to 5x5. I have a line in a '72 1/2 ton rearend but I am unsure of the ratio. The seller said that the axle tube serial code reads thtwo191. Does anybody know what that says about it????? I really need to figure out what the ratio is. I am hopeing to figure it out this way before I drive out and check it out and possibly pop the cover. I wunna know what I have and what I am talkin about before I pull out the cash.

Thanks any and all help.