Searching for info on Navy M1903 stock markings...

Hello Everyone,

I'm searching for information on some M1903 stock markings that have generally been attributed to US Navy issue. Specifically, the markings I am curious about are stamped block letters over numbers such as "AA 30" or "B 16" on the right side of the buttstock. My understanding is these are likely rack numbers of some kind, but where did this consensus among collectors come from that has linked them to the US Navy? Does anyone have documentation or photos that this was only a practice among Navy armorers? Another question I have along these lines is what about US Marines serving aboard Navy ships on sea duty? Do you think these Marines would have rifles marked differently than their Navy counterparts? Any thoughts?

Here is a photo of an early single bolt, high wood Rock Island 1903 stock. The markings on the butt stock are AT over 21 which is the US Navy designator for Ocean Going Tug boat and the 21 is for the USS Bagaluce in service from 1919 to 1946. Some early stocks may only have a single letter and it is difficult to I.D.

The other more common markings are the ship name like TENN for USS TennesseeAttachment 116618Attachment 116619Attachment 116620Attachment 116621