That is a nice collection, especially considering you say you found all of them in Washington State. The one poster was correct regarding the "Circle C" 41. They used regular copper nails in 38-41, but also used the copper nail with the "C" around the date, indicating use in a cedar tie. These cedar ties were usually yard cross ties & etc. and not mainline ties.

There are a couple of dates missing and a couple of varieties of dates that are fairly hard to find. There is an indented copper 27 that has "rays" around the date, sometimes call a "sunburst" 27. The reason for the rays or lines emanating from the date is unknown. Also missing are the 47 and 49. The 49 was used primarily in switch ties and was the last common nail used by the MILW. They did use a 50 and 51 copper, very sparingly, except for one location. The Kansas City yards. The Milwaukee Road was one of several railroad that co-owned the yards and alternated maintenance of that yard. They did the work in 50-51 and used their familiar little copper nails in that effort.

I will have to look more closely at your set of nails, but it seems like a real nice set.
Thanks for posting. If you have any specific questions regarding this set or any other please feel free to ask. If you want a general overview of date nails, the Railroad Tie Association has posted an article I wrote several years ago on their web page or just type in DATE NAILS in your browser, it and other items should pop up.
Regards,
Rolland Meyers