Holy smokes, that is beautiful! As you may already know, the Diamondback is built on the medium-small Colt "D" frame, same as the Detective Special and Police Positive guns. The barrels were made from the same blanks as the Python, though of course the Python was never bored for .22 except in prototypes. The Diamondback was the only "D" frame ever fitted with adjustable sights.
I sold a very high condition 4" .38 Diamondback a couple years ago for $1500, but prices have climbed significantly since then, and the .22s are rarer and command a premium, not to mention the Custom Shop engraving, box and long barrel, which make that gun a collector's dream.
As to value....I would expect to see that gun at a local show or gun shop with $3,500-4,500 on the sticker. You can also check completed Gunbroker listings (if you have a membership) for what similiar guns sold for.
In any event, it is magnificent. Thank you for sharing!
