I'm actually posting here in hopes of finding some more information on .fos files. In particular their structure and layout? I understand most of the header and then the two chunks of png data but after that it just looks like a mash of random save data.

Is there a documented struct for this file somewhere?

Thanks.

to be a little more specific i'm looking for information and/or documentation on change records within .fos files.

Okay, I will bite. There are no .fos files in the fallout directory.
Perhaps you need to explain more about where these files come from.

[EDIT] You mean save games?
Why do you need to know that? With a mod you should be able to do anything you want.

Yes he does mean save games, they end with .fos. (e.g. Save1.fos) I am also curious as to why you would want (or really need) to alter a save game. You can change almost anything simply by using the console.

Yes I'm aware I can change anything from the console, and yes you can change anything you want (I really wasn't debating either of those facts) this is purely just my curiosity and want to know nature.

To my knowledge there is almost no knowledge of the fos format. Similarly, there is very little knowledge of the ess format used by Oblivion. You may be able to gather a bit from the source code of Wrye Bash, as it did have a few functions to edit the save games.

@The naysayers:
Curiosity aside, I can list a number of reasons to want to manipulate the saves directly. Some problems (such as usage of setActorValue) cannot be fixed from the console. Another would be to address problems with bloat.

Thank you, I'll take a look but it's a shame there is not official documentation.