I asked MDWFP and here's the reply.
Thank you for contacting us at the Mississippi Department of Wildlife, Fisheries, and Parks.
Back when US Postal System changed the old zip code system in the 80's and the US Coast Guard changed their system, the two did not talk on the abbreviations on the different state names...That's how we ended up with MI instead of MS.
Thanks and be safe outdoors...
Sammy Fisher