Translation:Write more, sir!
Señor can mean mister as well as sir. I got it wrong but I disagree.
It can, but "Mister" is usually only used together with the person's surname.
Escriba: here, a polite command to one person. See the tips.
What form of escribir is escriba?
Imperative for usted.
Mister is not very formal. Write more mister, seems rude to me.