The T5577 is an emulator chip, T5577/EM4305 may mean that they are selling T5577 chips that come pre-configured in EM4305 mode, that would be my guess anyways
The T5577 doesn’t do a whole lot when it’s blank, which leads to people thinking something is wrong with their chip when all they need to do is write some data to it. To prevent this, I imagine some sellers just ship the chips pre-configured as something