(a) In Class 4-A shall be placed any registrant other than a registrant eligible for classification in Class 1-C, 1-D-D, or 1-D-E who is within any of the following categories:

(1) A registrant who was discharged or transferred to a reserve component of the Armed Forces for the convenience of the Government after having served honorably on active duty for a period of not less than six months in the Army, the Navy, the Air Force, the Marine Corps, or the Coast Guard; or

(2) A registrant who has served honorably on active duty for a period of not less than one year in the Army, the Navy, the Air Force, the Marine Corps, or the Coast Guard; or

(3) A registrant who has served on active duty for a period of not less than twenty-four months as a commissioned officer in the National Oceanic and Atmospheric Administration or the Public Health Service, provided that such period of active duty in the Public Health Service as a commissioned Reserve Officer shall have been performed by the registrant while assigned to staff any of the various offices and bureaus of the Public Health Service including the National Institutes of Health, or while assigned to the Coast Guard, or the Bureau of Prisons of the Department of Justice, Environmental Protection Agency, or the National Oceanic and Atmospheric Administration, or who are assigned to assist Indian tribes, groups, bands or communities pursuant to the Act of August 5, 1954 (68 Stat. 674), as amended;

(4) [Reserved]

(5) A registrant who has completed six years of satisfactory service as a member of one or more of the Armed Forces including the Reserve components thereof.

(b) For the purpose of computation of periods of active duty referred to in paragraphs (a) (1), (2), or (3) of this section, no credit shall be allowed for:

(1) Periods of active duty training performed as a member of a reserve component pursuant to an order or call to active duty solely for training purposes; or

(2) Periods of active duty in which the service consisted solely of training under the Army specialized training program, the Army Air Force college training program, or any similar program under the jurisdiction of the Navy, Marine Corps, or Coast Guard; or

(3) Periods of active duty as a cadet at the United States Military Academy, United States Air Force Academy, or United States Coast Guard Academy, or as a midshipman at the United States Naval Academy, or in a preparatory school after nomination as a principal, alternate, or candidate for admission to any such academies; or

(4) Periods of active duty in any of the Armed Forces while being processed for entry into or separation from any educational program or institute referred to in paragraph (b) (2) or (3) of this section; or

(5) Periods of active duty of members of the Reserve of the Public Health Service other than when assigned to staff any of the various offices and bureaus of the Public Health Service, including the National Institute of Health, or the Coast Guard or the Bureau of Prisons of the Department of Justice, Environmental Protection Agency, or the Environmental Science Services Administration, or who are assigned to assist Indian tribes, groups, bands, communities pursuant to the Act of August 5, 1954 (68 Stat. 674), as amended.