Last night I took the Suomi as far as was worthwhile with ever-finer grades of emery paper -- the last passes were wet-sanding with 400 grit, then a going over with 0000 steel wool.
There are deep scratches from the various machining and grinding processes that wouldn't go away unless I took almost a hundredth of an inch off. That, and there are remaining voids and pockets that aren't going to clean up anyhow. Plus some visible mis-alignment of several of the pieces. So this degree of polish would have to suffice.
Of course, it needs to look at least something like the other parts of the gun. I am actually not sure what process the Finns used on the originals -- probably a hot-bluing tank, but really could have been any number of processes. But rust never quite sleeps (it only grows lethargic) and the final look of the original hardware is a nice mixture of blue-black and rust brown.
After a lot of research online, it appears that at least some people have achieved a decent-looking period finish by combining Plum Brown with cold bluing. (Cold blue also doesn't wear well, but this is getting only small amounts of handling. It isn't as if anyone will be taking it to the range!)
The Plum is a strong acid. I degreased with hot soapy water and 70% alcohol, then put on a coat. Almost instantly the receiver turned a rich coppery brown:
I'll have to remember that in the future. It was also a remarkably even coat, especially considering the cap on the bottle broke and I got drips of the stuff all over the work table.
The Birchwood Casey Perma-Blue, on the other hand, went on very spotty. But I can't blame the liquid. I think it reacted with the layer of Plum Brown -- which is essentially red rust -- and perhaps with some of the water or even not-quite-deactivated Plum Brown.
But I wiped it on, let it sit for a few minutes, then rinsed in hot water. Rubbed off the loose material and buffed it up with steel wool, then did a second coat. This is where it stands at the moment:
The color match isn't bad. It needs more buffing out, and I'll want to go back with a Q-tip and touch up some spots that didn't get enough black. In life (as opposed to in photograph) it looks smoother, but a little lighter than the original part. But the final coat of gun oil is going to darken it, I'm sure. I'm also going to go back with steel wool and even emery paper to knock the bluing off some of the exposed edges -- weather it, in other words.
It looks like it is going to work. My major worry at this point is the treatment is too fragile and will get scratched up when I clamp into the mill to drill the last couple of holes.