Our military ran short of front line manpower in the late 1960s and early 1970s, so an old tradition was dusted off to get the right kind of talent into the right billets, within the constraints of the law. When I was assigned to be the Weapons Officer of USS Odax, I was a Lieutenant (Junior Grade) or O-2, in a position that called for a full Lieutenant, O-3. So I got promoted. At least, to all external appearances I got promoted. Unlike real life, where the general HR policy is to keep all personnel actions confidential, dates of rank and lineal numbers were well known among naval officers.
I got two official letters regarding my appointment as "Weps." The first was the letter informing me of my official duty assignment. The second letter is shown below. This process is traditionally and historically referred to as frocking, as in wearing the frock coat of your rank but not getting the extra pay.
So, like George Armstrong Custer, I was frocked.