I picked up a couple double barrels today. One is a 20ga with no model no. listed on it. The odd thing is the receiver seems to be a reddish bronze color.Is this a factory finish?
http://imgur.com/yjfVnnx
http://imgur.com/TfgLsSM
http://imgur.com/e68i1k6

I doubt it (that it is a factory finish). While good solid shotguns the 311 and variants were blue collar shotguns of that era and one didn't see color jobs like that in the local Western Auto hardware store gun rack or through the mail order catalogs of the era.

I'm going to guess that it was a refinish job and dealing with the cast receiver led to that color (?).

I'm going to guess that it is factory. During the 1950's/ early 1960's it was popular to use an anodized finish on "trade" guns to fancy them up in the eye of buyers of the Art Deco world that existed then. Lots of .22's came with off-color magazine tubes, or receivers of a "different" color. Your "Springfield" was a leader brand for Savage after its purchase of Stevens and would be the genre of gun on which they would use such a finish.

Many firearms with cast receivers get that plum color over time. It has to do with the content of the metal used in the casting. I have heard it is the inclusion of silica and I have heard it is the nickel level. Regardless, it often happens in cast parts over time.

The most glaring examples I can think of are Ruger firearms but they are not the only ones.

The plum is a defect and you'll see it on some Greener GP Guns. It means that whoever did the blacking didn't do the job properly.

I don't know what exactly causes it but I seem to recall that it is through re-use of the bluing mixture beyond when it should have been thrown away.

Or, as I say I can't recall which, something to do with not having the temperature of either the metal piece or the bluing mixture correct.

OTOH defect as it is some do like the effect that it gives.