Yes, you are correct. The magazine fed actions do not have the locking lug recess support at the 6:00 clock position that the solid bottom PTA action has. Info on this site indicates that Savage moved the magazine cutout rearward slightly for the 338 Lapua chambering. There is also some info regarding a different action heat treatment that I have not been able to confirm. My 300 Norma Magnum has the same bolt head as the Lapua and I take special precautions to alleviate and verify increase in headspace. One advantage to the NM case is that it does not have as much body taper as the 338 Lapua which equates to less bolt thrust provided both are operated under similar operating conditions.