How do you describe someone who is the next up in the chain of command for a spy, or several spies, if you are writing a novel which is set in the American civil war and involves spying?
The modern description is ‘handler,’ or ‘controller,’ but I suspect that these were not in use during the civil war; in fact I doubt if they were in use by any government secret service around the world during this period of 1861 to 1865.
Spying expressions in World War 2 and the Cold War
I believe, but can’t be sure, that they might have been in use during World War 2  and the Cold War, and the current age of spying.
But even that isn’t guaranteed; it’s quite possible that they are not used by any modern secret service – that they only used by authors who are writing spy novels!
Whatever the case, I have to describe someone who is the next up in the chain of command for a spy.
How to solve the problem
So, how to use a description which might or might not have been in use during the period which I’m writing about?
This is how I got around the problem (in fact I devised a couple of ways of getting around it.)
This is one way.
‘He was his controller, he supposed he could be called.’
This is another way.
‘He was his controller, he supposed was one way of describing him.’
Obviously, substitute ‘handler,’ if preferred.
Note the expressions ‘he supposed he could be called’ and ‘he supposed was one way of describing him.’
This way, the character doesn’t use the expression ‘controller’ as if he is familiar with it.
He can’t be familiar with it, because it is not in use.
Instead, he uses it as if it has only just occurred to him to use it.