Hey Joe,
I was able to download and zoom in on the photo you provided. The revolver obscures much of the text, but this is what I was able to see.
Serial number N629200, made or shipped in 1978, six years before the first 3" round but N frame, the 24-3 made for Lew Horton. More than likely a no dash model, sold for 80K at the time!! WAY, WAY outside of my league. The letter states engraved, I don't see any however.
Kind of reminds me of the three 3" guns commissioned by Dick Marble without the factories knowledge. Since this one is lettered, it must have had the factories approval. Although I can't possibly afford it, I would appreciate any additional information you might come across.
Best regards, Bob.