I’m having a bit of trouble finding some solid info on the specific brass alloy used for the reed plates. The basic understanding is that this type of brass was used for the reeds and reed plates until around the 50s. It was only made in a couple of factories world wide and eventually became not economical to make. Harmonicas only used a very small percentage of brass and the other consumers of raw brass went to a cheaper form. The prewar brass is thinner but more sturdy allowing for easier manipulation but also lasts longer. I could easily tell a difference when I got my first prewar Hohner in the way the reed plates reacted under hard and soft playing.
I’m currently building my collection (I’ve got about 12 now? I’m hoping to have around 30 of different keys and conditions). I want to make a fancy display for them and show them off at guitar shows. They may sell, they may not but I don’t care. I think they are really neat pieces of history and undervalued. You can’t get that kind of brass now anywhere. Your only chance is finding a good condition Marine Band or Old Standby. Plus, the boxes are very aesthetically pleasing and would look nice on your desk next to your vintage Gibson J-45. At least they look nice next to mine.