What is the deal with the names agents are referred to at S.H.I.E.L.D?
We have normal names
- Agent Coulson
- Agent Sitwell
- Agent May
We have code names
- Black Widow
And then we have numbered agents
- Agent 33
- Agent 13
Obviously Agent 13 and 33 aren't the 13th or the 33rd agents ever at S.H.I.E.L.D. Are these just code names used for undercover agents, and later people just keep using them? If so why is Black Widow her undercover name, and not a number?
Agent 13 tells Steve Rogers
"Captain Rogers, I'm Agent 13, S.H.I.E.L.D. Special Service" - Captain America: The Winter Soldier
You might say. "Ah Special Service, that explains it."
Except, then Agent 33's bio says:
Kara Lynn Palamas aka Agent 33 was a researcher and historian who has a special interest in classical mythology. - Marvel.wikia
She does not appear to be a member of Special Service.