I have an Eddystone m1917 with the following marks on the left side of the stock:

(boxed) RIA with FK underneath. I understand this to mean Rock Island Arsenal and the FK is the inspection manager's initials.

(boxed) 3 GM-K is underneath the (boxed) RIA FK. Several threads have mentioned that this might mean General Motors.

My questions are:

Am I wrong or right on the above assumption(s) and what is the 3 and K stand for?
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