Barrett, welcome to the board. The Merc TRS outdrive began being produced somewhere in the early 1970's. Merc discontinued the TRS around '92 or so. The Bravo drive is what began replacing the TRS in about the mid 80's. The TRS is much larger than the Bravo One drive and is a little stronger than the standard Bravo One drive. The TRS was a good drive---just outdated---plus it required a transmission set-up which made the Bravo One much more attractive because of it's cone clutch shifting that eliminated the need for a transmission and gave another bennefit of having more room in the engine compartment as well.
Perhaps someone else can jump in here and shed a little more light on the TRS for you.