I have purchased(not cheap...) four cartridges from an Hungarian seller, thinking by their aspect that they where some form of copy or clone from the original.
But to my surprise, they are completely original 16KB cartridges, both the case and the card inside:
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I have searched for information here and on the
Museum page, and have found that the "softcart©" range of products where offered at the Centrum store on fall 1987.
As you can see, they have a similar sticker than the EP64 cartridge but with the "softcart©" logo on the side.
Internally, the only modification is an Eprom glued to the socket.... thieves watching over their spoil.