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http://www.hummerknowledgebase.com/fuel/ftac.html
part number 57K3529 matches the NSN listed above, instructions come in the kit to my understanding.
HummerPartsGuy has them in stock, on sale right now.
https://www.hummerpartsguy.com/KIT-FUEL-TANK-ACCESS-CUTOUT_p_22336.html
Also found one...