I found it here!! Searching for Periodicals on this forum. The command is
mobi2mobi filename.prc --booktype "257" --outfile filename.prc
Works LIKE A CHARM.

Thanks whoever figured THAT one out!
Now I have about 600 books on my Kindle... anyone know how to set the booktype of all of them at the same time?
I wonder if changing the booktype makes the Kindle re-search them. It's been about 4 days since it had 500 files to go and it's at 100 files, so it's about 100 files a day that it can get through. Almost done!