How to keep it shinning
The present day Marine Band 1896 has stainless steel covers, which do not discolour through playing. You only need to wipe them down with a soft cloth. Older examples however have nickel plated covers, which lose their shine through contact with saliva.
as you are putting in your mouth, do not use any harsh cleaners like oven cleaner, WD40, ammonia. They also thin the plating making more pitting in the future. saliva on kitchen towel rubbed in circular motion keep repeating this hope this helps.
even this looks bad after repairs inside and out now happy to play.
To clean the cover plates properly, remove them from the harmonica. Even just rinsing them in warm, soapy water and a light brush will clean them up pretty well. Dry and wipe down with a soft cotton cloth or rag. Don’t soak the whole instrument in water. You can wash plastic combs. But do not wash wooden combs.
Then after using the instrument, a good way to clean and shine the instrument, is to use a silver cloth as used by orchestral/marching bands on tubas, cornet, horns etc. These cloths are made for silver plated instruments and are an excellent cleaning tool for harmonicas. Trevor James (flutes) and Denis Wick (mutes and mouthpieces) are two manufacturers who make these cloths. Both are excellent quality.
Finally, just wipe down the outside of your harmonica after playing and that will help keep your instrument clean and in good condition. It’s much nicer to play an instrument like that.
I hope that helps.