The top comb is not flat, the bottom one is.
One of the time-tested methods of improving the Marine Band harmonica is to flat sand the comb. This makes it much more airtight and removes the waterproof seal (on current Marine Bands only - the old ones don't ever have a seal) and can sometimes cause the comb can swell. It can become distorted and leak air, causing the harp to play poorly. The tines can also start to peek out and that can make the harp very uncomfortable to play.
This can happen in a week, a month or with a little luck can sometimes take years before the comb wears out. But this unpredictability means that the flattened, unsealed Pearwood comb is usually immediately replaced with a third-part aftermarket comb in most custom harmonicas.
So how can you experience the "gold-standard" unsealed, perfectly flat Pearwood comb? If you send me a Marine Band harmonica for Service, I will automatically provide you with a high-performance, flat, waterproof comb. For a very small extra fee, I will provide you with your harmonica's original comb, drilled for screws and flattened to my standards in addition to my replacement (long-lasting, waterproof) comb. You get the best of both worlds!
Compare for yourself. You may fall in love with the authentic tone of unsealed Pearwood. Or you may not notice a difference between the two - equally flat - combs. Either way, you get the choice.
More information on combs
The very early Hohner Marine Band harmonicas used Peachwood as their comb material until about the 1920s . They switched to Pearwood and have continued to use it to this day. Some feel that the wooden comb gives the Marine Band its signature warmth. Others feel that comb material has very little to do with the overall tone and that the way to get that warm tone is to make the harmonica airtight by making the reed plates and comb as flat as possible.
Brendan Power and Vern Smith used the scientific method at the 2010 SPAH convention and enlisted an army of pro players to test various comb materials on a single harp before a live audience. One comb at a time, players played the same lick before the audience; neither the players nor the audience were told what comb material was. Both the audience and players rated the tone and response of each comb sample.
The quick-switching mechanism was a point of conflict as some of the pro felt the test harp was not airtight and therefore the instrument's tone was not representative of real-life... One surprising conclusion was that despite the range of responses from the pro players testing the various comb materials, the audience couldn't tell the difference between comb materials. They could distinguish between players, though.
A similar event was held at SPAH 2013, but admittedly, that event was far from scientific. The audience and players had a lot of fun showcasing the product line of a harmonica comb company. That event successfully engaged the audience and got people talking about combs without as much vitriol as in 2010. But there can be no conclusions drawn from this experience due to the lack of standardisation used in the process.
So the jury is out as to whether the comb material really changes the tone of a harmonica.
To further complicate matters, no out-of-the-box harmonica has a perfectly flat comb. The Marine Band is a paradox because every single one ever made has the potential to be a world-class instrument - the one-of-a-kind favorite harp in your case. But none of them are set up from the factory to fulfil that potential.