The Camaro Club (UK)

So, I have to do some work on my Diff and am trying to identify what exactly it is.

Is anyone able to help?

These are the codes on the rear axle
Rear of diff drivers side: d289 - I think this shows build date April 2nd 1989
Rear of diff passenger side: 12845
Front of diff drivers side: 3894860NF

I can't work out the latter 2

The car is a 1969 Camaro but this is clearly an aftermarket diff?

Thanks all, I hope someone can help me identify it.