How to bulk edit Mail signatures.

The Mail signatures are nominally held as *.mailsignature files in ~/Library/Mail/V10/MailData/Signatures. I have successfully edited the *.mailsignature files to change the signature, so that the half dozen of so signature have a common motto that I can change frequently. It's not a simple process. The signature has a normal mail header with trailing blank line, followed by HTML encoded in quoted-printable (QP) form, to keep the length of the lines below 72 characters. Newlines are preceded by the QP escape character '=', and non-ascii UTF8 characters are also encoded in hex with a preceding '='. Editing involves decoding the QP of the HTML part, substituting the new text, and recoding the HTML part as QP.

I have this process working, and the results have been until very recently accepted by Mail, although it generally required that Mail be shut down while I changed the sigs, and left for some time before restarting.

This process is now not working.

Mail buffers the existing signatures somewhere. I can shut down Mail, modify the sigs, restart the computer, restart Mail, and–voila!–the old signatures have overwritten the files in ~/Library/Mail/V10/MailData/Signatures.

Any idea what's going on here, and how I can take control of my own mail signatures?