Systema TW5 Upper Receiver Material

[PoFF]

Ok, very specific question here :

I've got a nice TW5 Max, great gun, but my only disappointment is that, unlike the barrel/muzzle/cocking tube combo, which is blued, the receiver is painted, and it got scratched with use.

Now my question is, I know that receiver is made of steel, however, some places report that it was stainless steel, but some other places did not specify. My objective is to get the receiver sandblasted and hot-blued like the rest of the gun (got a Gunsmith who does wonderful bluing jobs), which would not be possible if it actually was stainless steel.

Any info on that matter would be greatly appreciated.