Rear Axle I.D. Confusion

[GMC_Aaron]

I was trying to pull the axle codes the other day, because I need to change the cover gasket. I have read about casting locations and couldn't find the code that is supposed to be on the passenger side of the rear axle. What gives? I was able to pull a couple of numbers off the axle. One was on the carrier itself, passenger side, facing towards the front and it read GM 30. The other number I found was on the driver's side axle tube, facing towards the front. Will these numbers tell me anything? I'm thinking that this rear axle isn't the original.