Dr. Holbrook's Military School was created in 1866.
It is not a military custom it's a military command, 'stand at ease' means to relax from the 'stand to attention' command.
Depending on how it's used, it could stand for "Military Specification", meaning the ability to withstand shock, dust, or harsh environments.
Military Specification
MPO stand for Military Police Officers.
That is the name of a person. The "dr" stands for doctor.
Drive
Doctor
Dr Military
It is the military term for a 'search and rescue dot.'
Doctor of Osteopathy
dr stands for doctor