@liaizon @neauoire I'd love to find out some day, but don't have the time any time soon to rip one apart and RE it with an Arduino or whatever even if I found a free one.

Looks like the newer C-Map carts are actually just encrypted / DRM'd SD cards with a PC app to update. I believe Navionics hardware carts are the same general idea. These really old SNES-cart looking ones? Unknown.