Tom Mix was an early film actor starring in many a (mostly silent) Western.
He has become Cockney rhyming slang for the number six. Rhyming slang seems to enjoy messing with numbers as we shall see.
If Clint Eastwood had been a cockney, Dirty Harry would have been that much funnier.
"This gun has Tom Mix bullets, I may have used man alive, I may have used Tom Mix...."
That would have confused the baddies.