Apologies but I’ve been away for a couple of days and just seen this. To the best of my knowledge the original stand alone version is the first link. I’m not aware of any earlier ones and note that PHoD credits the D20 Modern project as the original source for his re-textured version.
It was developed by one of the then D20 Modern team as part of the hak set then incorporated into the D20 Modern system hak packs for version 1.3/1.4 and is currently still included.
Credits for the original should be available in the documentation. However, it has been subsequently worked on and improved on a number of occasions, not least most recently by Vahnhaunt who introduced a great many changes.
I can’t see there being a problem with your adapting it, it’s marketed as free for use as long as your own project is free for use, but I would like to request that you credit the original source and Vahnhaunt for his updates.