Saving portions of a large file - theoretical problem

Lets say you had a large file (e.g. 500mb big). You were told that the following sections of the file had been modified:

0mb - 1mb
1mb - 2mb
50mb - 51mb

And you wanted to copy those blocks of data out of the file, saving it as a backup somewhere that is about 3mb big.

You're told that at some point in the future you will have to restore this file based only on a) the original file, and b) the blocks that you saved. You will have to merge them to recreate the file from it's two parts. When the time comes, you won't have any special premade tools to do it with, short of maybe a readme.txt that you wrote for yourself. You'll only have regular windows or linux commands and software, and whatever you can easily google up.

If you knew this situation would happen, and you could store those 3 megabytes of data in any format you desired, how would YOU go about storing the blocks of data such that you would be able to restore it later?

I'm trying to explore the idea of backing up only portions of a file, but doing it in such a way that any relatively knowledgable person could restore the backup without any special software. It is kind-of a theoretical question, not looking for any concrete solutions, just some ideas.