No, I suspect that 11B used, was "picked" when he set up his profile. Broom's Primary MOS was 9305-Psychological Operations Officer, his assignment in the 4ID was 11B.
The Infantry MOS used at the time:
1542 Infantry Unit Commander
1543 Infantry Heavy Mortar Unit Commander
1560 Infantry Unit Commander (Mechanized)
Suffix Description
3 Special Forces
4 Army Recruiter
5 Nuclear Weapons Employment Specialist
7 Parachutist, anyone awarded jump wings upon completion of Airborne School
8 Ranger
http://www.ichiban1.org/html/history_mos.htm
SO, A qualified SF officer was a 31542, or could have it show as a Duty Assignment, even if you were not qualified.
Forgot to add. In Vietnam, Special Forces Officers and NCO slots were filled by non-school qualified. There were inf, arty, eng schooled troops in the SF slots. Sometime in the 70t's the reg was changed so that they could be paper qualified as SF, if they met the criteria.