@Felthry I have no idea. The spectrum did typically use tapes, and they were the main format, but the old rubber key, 16k spectrum had the ZX Interface 2, which Sinclair themselves made, but there were also competitor Interface 2 devices, like the Ram Turbo Interface.
@Felthry The original Interface 2 expansions for it had a rom cart slot in them as well. By the time of the +2 though, which had 128k, and the 48k models, the rom cartridge format had largely been deprecated I guess. Certainly the +2 and related models had built in Interface 2 controller ports but no Rom cart slot in them D: