Have made a few Mlers w/ keys and just use a metal/wood fit and haven't had any trouble w/ the fits later on.....my first LR had keys , was built in 1976 and the key fit has never been "adjusted" How were the original Hawkens made... I think they were very simply made asre the keys and the fit.
The "keyhole" slots are carefully laid out on the stock and a series of small drill holes removes a lot of the wood and these holes are located lower to leave some wood for filing the key/lug fit w/ the slot's upper surface. The webs between the holes are cut out w/ a narrow chisel and a thin needle file{could be burned out w/ an undersized key} ......I always take a little more off the slot's bottom surface {an escutcheon hides this} and then file the slot's upper surface for a nice press fit of the key and bbl lug.
The escutcheons {called "inlays" henceforth} are made but the slots of the inlays for the lower forrestock are different than those of the upper forestock. The lower forestock inlays {2} have a slot barely larger than the key which is used to locate the inlay for inletting. Hawkens have 2 per side like this.
Inletting and making the inlay slots for the upper forestock is more complicated because of the forestock's angled, rounded surface. I make the slots narrower than the key by .005/side and when locating the inlay for inletting, the upper surface of the inlay slot is lower by approx. .012 than the slot's upper surface in the stock. Inletting the inlay on a slanted surface causes the inlay to move upwards. After inletting, the slots in the inlays are filed using the slots in the wood as a guide. The slots in the inlays end up being at an angle to the inlay. As was said....the keys should not bear on the inlay slots.
I use pins made from paper clips to fasten the inlays and the pins have a ctsk head and raised barbs on the dia.{ a very small amount of epoxy in the holes can also be used.} After the pins are in, the heads and inlays are filed smooth. This is one method for installing and making keys and inlays and no doubt there are others. Sorry for the long post.....Fred