WASHINGTON - The next round of frontier AI models will stay in a padlocked steel shipping container at a bonded customs warehouse in Newark. British government testers may reach them only through a single brass letterbox slot cut into the container door, the Office of Transatlantic Model Custody said Friday. The setup follows reports that U.S. officials asked OpenAI and Anthropic to withhold new frontier releases from British government evaluators until a domestic security review is complete. OTMC, attached to the Commerce Department, said the container was a compromise that keeps the models on U.S. soil while meeting a testing commitment with London. "The British have unrestricted access to the slot," OTMC spokesperson Helen Marsh said. Under OTMC Procedure Notice 19-B and Customs Form CBP-7741-OTMC, the slot measures 3.5 inches by 0.75 inches (89 mm by 19 mm). Testers may post one handwritten prompt per business day. Replies come back on thermal fax paper slid through the same opening. Prompts that will not pass the aperture must be rewritten on Form OTMC-19B Continuation Sheet A, folded once along the long edge, and resubmitted the following business day; staples and adhesive tape are not permitted. An unnamed British evaluator familiar with the process said the queue was workable for short prompts and that longer test scripts were being broken into successive daily filings. OpenAI and Anthropic said in a joint statement that they were "fully cooperating with the slot."