Abilene,
I think I agree with you. It looks like there may be a coating on the receiver. Sounds like a good cleaning with a lacquer thinner like the Reverend suggested might be called for. Or you could try a citrus solution like De-solv-it . Don't use ammonia.
Tayhitman44,
Actually you can speed up the process, I had someone wanting their receiver to look like it was aged and they asked have some of my BP fouling I pushed out of my bores and they made a solution with some water and painted it on a little at a time.
It will look more like Johnson Barr's than mine. Mine have the pattern from burned/burning black powder scattered from racking an action fast and smoking shells coming out.
Note, the carbine in the second picture has different patina on the side plates than the receiver body. I actually took readings with a handheld LIBS unit and the side plates are a different alloy than the body. The original Henry and Winchester receivers were not Brass, they were Bronze, which was called "gun metal" at the time. Bronze can be very strong and was actually stronger than some of the faux steel (more iron than steel) the early pistols and rifles were made from, especially if they were castings. The Uberti carbine is two different high strength Brass alloys with very little Tin in them which would make them Bronze. I think the rifle in the picture above it also has different materials on the side plates looking at the patina difference but I didn't measure that rifle.
What caliber and barrel length did you get? As you can probably tell I have a things for '66s.
~Mako