Please help identify this A/C unit.

[prostreetC-10]

So my next door neighbor tells me he dragging home a free '70 C-20 that hasn't ran for 15 years. Is mom-in-law just gave it to him. Was a farm truck and always parked in the barn. Virtually NO rust. Killer patina. Speedo says 49*** and we are thinking that's actual given it's history. I come walking down the sidewalk and the first thing I notice is "8-400" on the fender. Yep, he got himself a factory 396 truck. Extremely complete and unmollied.

Here is the stumper: It has A/C that isn't factory (not an A/C cab). What we are trying to figure out is if it could be a dealer installed item or completely aftermarket. The engine part looks pretty stock but in the cab looks like A$$. The unit doesn't sit flush because the air box is so big in the cab and has to be pointed almost straight up. The compressor bracket is huge and has TWO idler pulleys that you can barely see in the pic. What do you think it is? Thanks!