What’s the origin of the mysterious phrase “etaoin shrdlu”?

Dear Cecil: You are my last hope. I need to know the origin of the phrase “etaoin shrdlu.” I first encountered it in Walt Kelly’s “Pogo” comic strip, where it was the moniker of an irascible bookworm, and it has turned up periodically since. I have searched, in my own pitiful way, for any printed reference to this enigmatic jumble of letters, but to no avail. The nearest thing I have to a lead is the guess of an old pressman. He suggests that “etaoin shrdlu” is what they used to test Linotype typesetting machines with. Apparently the letters e, t, a, o, etc., were placed in two rows at one end of the keyboard. A typesetter who wanted to test the machine would run his fingers down these rows and rattle out the nonsense phrase. Can you confirm this? John S., Chicago