MH is the name of a Unix MUA (a rather old one, at that), the name of the format it stores its messages in, and finally an ISO3166? CountryCode? for the Marshall Islands.
The MH MailBoxFormat? is a simple example of MessagesAsFiles?, where each directory is a Mailbox, and each file is a message. Filenames are integers which are incremented as each message is added to the mailbox.
There's not much else to it, it's a very simple (and hence convenient) format.
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