@kwmsmith : Summarizing our in-person conversation:

1) It's the same length as NumPy's magic prefix (6 bytes), letting me reusing the some of the `.npy` code from NUMPY more easily.

2) It starts with a non-ASCII character, as recommended here:
http://www.fadden.com/techmisc/file-formats.htm
so as not to be confused with a plain text file; I'm guessing this is the same reason the `.npy` format uses it.  I'm not sure whether it's a good or bad idea that I used the same first character as `.npy` files.

3) The 'DARRY' part; other than being 6 characters, it's just the best I could come up with.  If you have something better, let me know.