It's definitely a Signature, even though it probably wasn't called that when it was made.

From Davidson's beginnings in the early '70s, right up to about '84, there weren't named models really, almost all the bikes were custom. It was only after the debut of the lower-cost production models (Challenge, Discovery, Impulse et al.) that the custom frames got a model name. They just started calling it the Signature since every one had a Bill D. signature decal on it. Never mind that a lot of the other production models got sig decals too, so the logic is not airtight on that, it's just how it is.

This frame has Otsuya lugs, and seatstay tops that are pantographed (later ones that mimic the pantograph look were actually investment-cast with the logo in). Those two things narrow down the date it could have been made, a little, but there's no serial numbers and no records, so you'll never know the date precisely. I'd guess '82 to '85.

I love Otsuya lugs. I made my wife's and my own light road frames with those.