Much of Hertfordshire became a large military training camp when the First World War broke out, and large numbers of soldiers were billeted in private houses. When I researched the book The London Gunners come to Town in proved hard to establish which units were posted where, because the relevant records had not survived. Much of the surviving information is scattered and the envelope illustrated here provides a named soldier, a regiment, a named address in Berkhamsted, and a date. The page Billeting in Berkhamsted in the First World War provides more details.
In addition I will use the page to add other soldier - regiment - billet - date information I can find for Berkhamsted - including any information you can add.