It is my understanding that the "bolster" as you refer to that reinforced receiver area came about when the U.S. Army ordnance board went from the .30-Gov't-03 cartridge to the adopted version in 1906-- the .30/Gov't-06.. Why, your guess is as good as mine. have a Model 70 in .257 Roberts-- made in 1947- I'd like to have a fiver for every coyote and fox and raccoon I have killed with it--excellent varmint round indeed-- RWTF

"The field is the touchstone of the man"..