I've been using resources such as http://www.uesp.net/wiki/Tes4Mod:Mod_File_Format to ascertain the tes4/ESP file format for Oblivion & Fallout 3 with the intent of writing an interior generator. I've had some success: for both Oblivion and Fallout 3, my program writes valid ESPs with multiple interior cells that contain references to statics.

The problem for me right now is teleporting doors in Fallout 3. I have been able to write valid teleport door pairs for Oblivion ESPs, but not for Fallout 3.

The ESP file format differs between Oblivion and Fallout 3. There are no references or docs for the Fallout 3 variation, so I've had to use trial & error to get things to work.

I've noticed that record lengths differ in fallout 3 ESP files. For example: REFR records contain an extra 4 bytes. The extra four bytes are always 0f 00 00 00 (four byte Intel-order integer for the value fifteen).

Getting to my actual specific question ...

GRUPs are 20 bytes in Oblivion, yet 24 bytes in Fallout 3. Looking in a hex editor reveals that the extra bytes are at the end of the record and that they contain what appears to be randomly variable data. It obviously can't be random, I just don't understand the pattern.

Does anyone know what is going on with those extra four bytes in the Fallout 3 ESP GRUP record?

Thank you in advance.