What sort of “nonnamous” letters does Tom write? Why?

Tom writes "nonnamous" to throw everyone off, so they can rescue Jim.

The warnings....

Beware. Trouble is brewing. Keep a sharp lookout. Unknown Friend.

Don’t betray me, I wish to be your friend. There is a desprate gang of cutthroats from over in the Indian Territory going to steal your runaway nigger to-night, and they have been trying to scare you so as you will stay in the house and not bother them. I am one of the gang, but have got religgion and wish to quit it and lead an honest life again, and will betray the helish design. They will sneak down from northards, along the fence, at midnight exact, with a false key, and go in the nigger’s cabin to get him. I am to be off a piece and blow a tin horn if I see any danger; but stead of that I will baa like a sheep soon as they get in and not blow at all; then whilst they are getting his chains loose, you slip there and lock them in, and can kill them at your leasure. Don’t do anything but just the way I am telling you, if you do they will suspicion something and raise whoop-jamboreehoo. I do not wish any reward but to know I have done the right thing. Unknown Friend.

Why nonamous letters?

So Tom said, now for the nonnamous letters.
“What’s them?” I says.
“Warnings to the people that something is up. Sometimes it’s done one way, sometimes another. But there’s always somebody spying around that gives notice to the governor of the castle. When Louis XVI. was going to light out of the Tooleries, a servant-girl done it. It’s a very good way, and so is the nonnamous letters. We’ll use them both. And it’s usual for the prisoner’s mother to change clothes with him, and she stays in, and he slides out in her clothes. We’ll do that, too.”