For skill points I do it a bit differently. Because it represent things you have learned over a period of time, if you have had the item for a whole level, and have worn it a majority of the time, when you level you get extra skill points. This represent the easier time you had learning those skills during your last level. If you lost the headband, then you would still have the skills that you learned (Learning a skill, is much different from memorizing a spell and keeping it in your head. That is why it only happens once a level, not one a day). However if you lost the item, when you go up again in level you would not get the points, as during your level you went back to your old intelligence and things got a little harder to learn. As for Spell Mastery, while trying to think of it the same way, I would say that your intelligence for the last three levels would affect it. But, that is way too complicated to try to figure out, and in the spirit of keeping the game fun and moving it along, I would have to say it isn't retroactive, just by looking at the wording. The number of spells you master are dependant on your intelligence at the time. (though I might be willing to let it apply retro, if the player really wants, and it isn't a balance issue. I don’t think it make that much of a difference to try to rationalize one way or the other, so I would go with the easiest decision.)
