Here is what Surgeon has to say about that.......
The WSM action has a longer loading port cut in the bottom of the action and bolt stop to match, and the internal feed lips of the action are widened. The purpose of this is to allow the use of extended internal box magazines like Wyatt’s. (So if you are going to be feeding the rifle from Surgeon DBM or any other system that uses the AICS mags there in no need to order this action you can use the 591/R and upgrade to a .550 bolt face bolt).