David and B-run.
The Mercs and the Mariners are built on the same line (2 Strokes only). The 4 Strokes however are not built on the same line. They are built by somone else. Or they used to be(as of 97). My 9.9 Yamaha is a 97 and the Mercs Are exactly the same! The TR1 on my Yamaha fits the Merc also. Same carb. same mounting plate ect. Check into it or lift the lids on both. I think you'll be suprised. Its cheaper for those companys to contract out the motors rather than tooling them up. Unless recently the manufactures have tooled there own. Sales are skyrocketing so its probably justifyable. Let us know if you find anything out. Easy on my spelling...

Bob D