M2 unmarked stock with hidden plug under buttplate

OK, this is something I cannot find ANY info on. Several years ago I purchased an M2 stock just to get the hardware on it. When I removed the generic looking buttplate I found a plug at the top, with glue or epoxy on the end, comvorming to the pattern on the rear of the butplate. Curious I removed the plug, found a finely milled out tube under the plug. Looks like it made to disguise the tube. Imagination going wild, hid documents, drugs, etc. Work in the plug is so well done that with a sling in the well you would almost never see the plug. Ideas, info, anything??? Not for sale at this time.