The finish on Flyer steamers changed through the years. The earliest ones had a nice flat black finish but the later ones got more of a satin, almost glossy finish. Your 321 should be of the earliest (and best looking, IMO) finish, meaning it would originally have been flat black. Many of the products offered as "Train Paint" are too glossy to match the early steamers.
The absolute best paint I've found for the 1946-ish locomotives is ACE Hardware Instant Drying Lacquer #17029 Flat Black. It's inexpensive and easy to find. Don't mind the fact that it is lacquer - it dries almost instantly and will not craze the original finish. You can try it on the underside of the pilot if you have any doubts, but I think you'll find this paint to match the original well. I have even used it to "touch up" places like dome tops and edges where the original paint has rubbed through and found it to blend almost invisibly with the original finish. And, if you don't like it, the other Good News is that it goes on in a very thin coat that will not hide detail, and actually dries (unlike lots of enamels). So, if you really don't like it, you can paint over it with the finish of your choice.
Try it. I don't think you'll be disappointed. Good luck.
