A patcher I created in Max 6.0.8 (Mac) that contains Message objects when opened in Runtime 6.0.5 (also Mac) can't find the Message objects, see attached image. I tried it on three different computers and got the same result. Updating the Runtime to 6.0.8 fixes the problem, but I wouldn't think there should be a difference between the two versions in regards to the Message object. It's also really a pain for distributing people who may not have the very latest version of Max 6 Runtime.
I’ve had similar issues in the past with an object from a newer version of Max not working on a slightly older version of Max Runtime. In each case, the main version number was the same, only the .x number was different.