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.
_________________________
Captain David Longenecker, CFI, CFEI
Fire Marshal, LCBF (retired) IAFF L-319
BSA Eagle Scout - Class of 1980
R.K.S. #1691; R.K.C.C. #CM83
N.R.A. Patron Life Member