you are right, it looks like the assembly has changed shape (due to the change in moisture content). the reason that happened is because finish has not been applied to the inside of the drum case, and only applied to the outside of the case. this causes a moisture imbalance in the wood, depending on the season - dry or humid. my guess is that it is in a dry spell, shrinking the inside while the outside remains stable.
personally, i would not cut or sand "to fit" yet. i would try to expose the pieces to a humid or moist environment
maybe with a humidifier in a box, bag or can. and see if that opens up the case. do it carefully and check it often, as you don't want to worsen the situation. note, if hyde glue was used, you may experince joint failure.
Last edited by TimPa; 05-13-2020 at 02:32 PM.