It is the catalog number for the Michel catalogs (German). In the US, we normally use SCOTT catalogs (SC#). In the UK and other British Commonwealth areas, they use the Stanley Gibbons catalogs (SG#). In France, they use the Yvert & Tellier catalogs (YT#). Unfortunately, each of these catalogs have unique numbering systems, so it must be specified which catalog you are referencing. If I would have just stated #84, you wouldn't find your stamp there in a SCOTT catalog or a Stanley Gibbons catalog, so I specified MI# so you would know what catalog my information was coming from.
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