Dana 44 rear axle? Stock or not? Please help!!!

[Griffin]

I have a 57 chevy 1/2 ton that is pretty damn stock. The one thing that confuses me is that it has a Dana 44 rear axle. i've looked all over the board at other people's truck and haven't seen one like it. It's a 6 lug Dana 44, the right width, with stock looking spring perches. was this ever an option on the 55-59 trucks?????????????????? is it just someone else's swap in????????????? Everyrthing else is bone stock.