This isn’t a boxlock, it’s a bar action hammer gun, a sidelock by default. It looks like a typical old Belgian barn gun, value is minimal but hey, you have to start somewhere. The most effective and least damaging method would be to remove all the wood and then completely immerse the action in a good rust dissolver, then after time as various screws lose their grip they can be loosened a little a day until they can all be removed, then the gun can be disassembled for a thorough cleaning. Caution! Break or strip one screw and you lose. Your best friend here is patience. You need to get a good penetrating rust solvent applied to all hinge points, locking and bearing surfaces. Many of these are not accessible from the outside so the liquid must soak or wick in from the outside, most effectively done by soaking. But soaking here can damage or destroy the wood. I have successfully used kerosene for this, others will have other things they prefer. This job takes patience, if you can’t wait a week or more between gentle attempts this job isn’t for you. Good luck.