Attachment 52183Attachment 52184Attachment 52185Attachment 52186

It would appear that some units either marked the buttplate tang (not as common) or marked the rear part of the stock

Ed Byrns translated these markings from a early 1903 stock;

Troop H
8th Cavalry
trooper #18
The 8th Cavalry, Troop H fought in the 1916 Mexican Punitive Expedition

Your 50-70 marks should be able to ID the unit