I am trying to restore a late 1800s desk that has a Yale cylinder lock (4 pin) with "S.251" stamped on the side of the cylinder. The curator of the American Lock Museum found an old Yale catalog with an entry for a S.250, which looks very similar. It calls for a 'no.10' key. I took my lock cylinder to a couple of local locksmiths and a Y6 key fit.

Two questions:

1) What is the correct key blank? Is it a RN-10 PARA, a 10-1/2 Yale, a Y6, a 997X, or ?
2) Where can I find a vintage key blank? I have looked all over the internet. My understanding is the key blank should have "The Yale & Towne Mfg Co" stamped along the circumference of the circular part of the key blank, along with "YALE" horizontally across the circular part.