Guess the particular reason for the shield containing the 15 vertical stripes will remain among one of those items lost to history..
The timeline though, in connection with the usage on the shield for the RCS and CG cap insignia can be pretty accurately placed around 1908/09.. I finally found the 8 x 12 glossy photo of the 1908 Master at Arms rating badge that is in the files at the Coast Guard Museum Northwest, at pier 36 in Seattle. I dug out the magnifying glass and gave a detailed look at the silver bullion shield that was the MAA specialty mark.. It is very detailed embroidery and you can clearly make out 15 vertical stripes (pales) on the shield. The 13 stars can also be distinguished.
The differences on the CPO anchors over the time line. Probably just a manufacturers choice.. Interesting though, the officer and warrant officer shield contain 13 pales, like the national shield.
For me, I think this little oddity is set aside until something official ever gets discovered... Been an interesting chase..