Who knows how many years this rusted beauty has been bolted to the chain-link fence of this school playground on Warren Street in Cobble Hill. At least since the bad old 1970s, I'd had to to think, the dangerous South Brooklyn depicted by Jonathan Letham in "The Fortress of Solitude." You have to appreciate the variety of font in the sign, though. I count six separate styles of type, the most remarkable being the diagonal, block-letter "Neighbors" slanting over the high-minded, urgent cursive of "Co-operate!" And how about that spiffy exclamation point. It's all so quaint and naive, I could cry.