The TT600/XT600 uses a different manifold from the XT550. The 550 might work on the 600, but not the other way around.
I think your going to find that the XT600 carb is going to be jetted a bit rich for the 550 mill too.
I got a perfectly working 550 carb I'll trade you if it is. Its to lean on the TT600 mill in my 550.