Stanley bailey no 4 dating
The format used for the type discussions and trademark designations generally follows other published studies. This is the Type Study part of a larger paper of 8 pages that also discusses the history, development and characteristics of this, the best plane ever made. See Beds marked with Bailey model numbers, No.2 to 8.
That bluing that Patrick Leach is talking about look like a mat greyish blue coating.
The original type study doesn't mention it, but there are several treatments of the lever cap, where its finish and the background color of the notched rectangle follow what seems to be a 'style du jour'.