The 1917 Mossberg catalog show 15A (wooden box) and a 15B (fibre case). I don't have a catalog that shows the metal box set. The 1927 catalog no longer lists a No. 15 set.
Not that the listing states the oval socket is "for three crank shaft bearing bolts" also the 17/32" and 19/322" sockets are tapered.
(http://i222.photobucket.com/albums/dd3/buswrench/message-board/mossberg-15-wrench-set-1917-1.jpg)
(http://i222.photobucket.com/albums/dd3/buswrench/message-board/mossberg-15-wrench-set-1917-2.jpg)