Here is a pic thru the mag well ("cleaned up") of one modified...
Note that the modified bolt is narrowed in width for almost the full length of the mag well opening, just starting to "flare" towards the original wide width before it is covered at it's rear by the action.
This is what is required in order to gain a ability to use a detachable mag that CAN BE PLACED/REMOVED IN ANY CONDITION (loaded or unloaded)
-as with anyother modern detachable magazine accepting weapon.
This is major SKS magic that I can't understand why anyone capable would not do, except the most dyed in the wool "purist" types. I would not use detachables without doing do, as w/o such, they are a PITA & have less reliability ('cause they get their flimsey selves bashed around too much in use).
Since it has no effect on use of the standard factory 10 round fixed box mag -it all come under the "Why not...duh!" catagory. Of course, screwing the bolt up if not done with care *is* not good.