Qualifications of staff officers; tenure; vacancies.

It is hereby provided that staff officers, including officers of the pay, inspection, subsistence, and medical departments, appointed in the National Guard of the District of Columbia, shall have had previous military experience and shall hold their positions until they shall have reached the age of 64 years, unless retired prior to that time by reason of resignation, disability, or for cause to be determined by a court-martial legally convened for that purpose, and that vacancies among said officers shall be filled by appointment from the officers of the National Guard of the District of Columbia.