all the MM had a tag on the inner fender that list the body type and the number for the car. that number was a production number. the 68 I have is a combination or like John said duplex it would be limousine style with removable panels. it could still be a duplex but not have the glass then it would be a landau style. they built straight hearses and duplexes this way with or with out removable panels. ambulances were built in the Limousin style with different roof heights. the tag for this 68 list it's production (model) number as 6868. lot of conversion the meaning of this number. the excepted one is that this car would be the 68th built in anybody style in 68 modale year. the model year would start in September of 67 so this car would be a build date of summer of 67 if that holds true. superior used a different tag and numbering system. there's was usually on the cowl. here are pictures of my 68 set up both ways and the MM tag. the serial number on this car is the same as the GM number on the cowl.