John's right about the DAO trigger.

Also, something else to keep in mind is that if you compare machined triggers from a 59XX and a 391X, the single stack 9 triggers have a thinner trigger (side-to-side) than the double stack guns originally did. (Well, the 59XX/69XX guns we got in '89/'90, anyway.)

The MIM triggers are uniformly thin, as I recall.

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Ret LE Firearms inst & armorer