I was referring to the storage compartment in the tang of the Mod. 17. On page 178 of Bob Gaddis' book you can see it. (The catalog calls it a slot.)
It remains a clever idea but not terribly practical in the 21st century.
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David Longenecker R.K.C.C. #CM83 RKS#1691 N.R.A. Endowment Life Member Residing somewhere in the Ubiquitous States of Amnesia