Mitch - sounds like you are also asking what makers used them historically, and how early?
For historic use, I know there are a number of examples. I recall at least one of the "Step Toe" guns illustrated by Wallace Gusler in his Muzzle Blast articles - I beleive it is the MESDA gun attributed to Botetourt or James Headwaters area of Virginia - has the vent pick holders (staples) under the cheek. It appears to be very early as far as surviving iron mounted southern rifles go, ca: 1790s. I recall seeing some North Carolina rifles with vent pick holder inlays as well - including the one on Mel Hankla's "American Historic Services" website(?), which is believed to be 1790-1800 era.
Guy