Some are, some aren't.  They were good horns when they were made in the Conn factory in Elkhart, Indiana.  Those are the ones that have "Elkhart, Indiana" engraved on the bell.  They were like the Artist horns from the same era, only with less sophisticated keywork.  After about 1960, production of 16Ms was farmed out to Best Manufacturing in Nogales, Arizona, and to Mexico around 1970.  The quality of the Nogales horns is lower than the Elkhart horns, and the Mexi-Conns are famous for poor craftsmanship. 


The keywork on the 16M is dated when compared with horns with Selmer type keywork, which may or may not be important to you.