I would like to know the technical reason why I am using manchester coding and $55 as the preamble.
Well, this is sure a strange question. I would think you would know why you are using it, since you have it in your code. I think the link rhino provided is good. But if you absolutley must know and have no clue why it is in your code, I would look at http://www.ottawarobotics.org/articles/rf/rf.html.

I know that it is working and that it doesn't really matter but I want to check my oscilloscope readings correspond with the correct convention.
Another strange one, how about using MPLAB (or the simulator of your choice) and see how the code runs. Then, compare this to what you see in the oscilloscope and see what happens. They should match, and you should be able to tell how the bits are done and how the code treats them.

I agree with skimask, I do not think it matters (as long as it is the same in both TX and RX).