I'm not familiar with "meyd-421" as a standard term or code. It's possible this could be:

1. A specific model number or part designation for a particular product or device

2. An internal code used by a company or organization

3. A typo or misspelling of something else

Without more context about where you encountered this term or what field it might relate to, I can't provide much more information. If you have any additional details about the context or source of this code, I'd be happy to try and help further. Alternatively, if you meant to ask about something else, please feel free to clarify your question.