Wait ... let me apologize. After more investigation, it turns out the two pieces I was given were given to me reversed. So, when I concatentated them in the order I was told, the fix up piece (the ugly part) ended up seeing just the end. It appears to be working now, but it's huge so it will be a while before I know for sure.

I should have had a clue ... the piece I was originally told was the first one was smaller than the second, and the second ended on an even byte boundary.