I’ve not opened up a marine band harmonica in a long time, as I use mainly Special 20’s. However, i know they used to use brads, which make it near impossible to get your reed plates back on in an airtight fashion. That is without replacing the brads with screws, which involves drilling and tapping new holes..
If you want to attach (upload) a photo or two, we’ll be able to tell you what they are, hopefully screws! or maybe someone else knows?
Having said that, I only recommend opening up your harmonica as a last resort, and in general just following the tips in my above post.