Almost.

The trick is the missing character is static, in the 'decompressor'. It's inserted between every segment which was trimmed to end where that character existed in the original.dat file. The numbers at the end of each segment file correspond to the segment number, as bourne shells increment numbers. It does not handle errors or validate the output.

It does fulfill the discussed terms of the challenge, which fail to include any reference to an external set of rules.