1971 GTO Hardtop rearend

I bought a 71 GTO Hardtop last spring. 400 ci, 3 speed automatic. Black on Black. I need to know what is the stock rearend for this car? I just had rear seals done and my mechanic said it has a Bonneville rear end!!!! Also how do I tell what rearend it has, and what is the difference between the GTO rear end and the 71 Bonneville?

there is no way you could have a B or C body rear end in an a- you have the right rear- now you need to find out what it is- a 10 or a 12. I can help you out if it's a ten bolt.