Beware, REND is really common (chunk headers are 4 bytes, H is “data”), but not always there, I have one example starting as “BLENDER-v279GLOB” and remember seeing “TEST” too. The part you want is just up to the 3 numbers (first 12 bytes).
You may want to check doc/blender_file_format/BlendFileReader.py, BTW. Some parts are what you want, ignore the rest.