That is really weird. I have the wide-screen version. I went to the scene you are talking about and saw pretty much the same thing. The difference is that it wasn't on his back....it was in lower left corner. It also wasn't 506....it was 502. It only pops up for a frame.
I tried searching around on the internet and couldn't find anything. The only thing I could find is someone asking about the significance of the 506 at the beginning of The Matrix. At the very start of the movie when it is showing the code the number 5550690 comes up and the camera zooms in...the number 506 is finally fills the screen before the camera ends up going through the 0.
Who knows what any of it means.
