Hi Dana.
I keep looking at that barrel band trying to visualize a connection with the rest of the gun. LOL Still scratching my head. Hmmmm. The wide shape of the band, and the style of the half-moon very much resemble the Turkish equivalent. See photo of similar on one of my pistols. Possibly the owner of the gun resided in Spain, but was from Turkish heritage? Of course I'm just speculating here. To me, the gun looks entirely Spanish made. It's only the barrel band that is a real curiosity. But also adds an interesting mystery.
Something I forgot to mention earlier: The pan on the lock may not be brass. It may be iron but gold plated (fools gold) which was a fairly common feature for higher end private purchase guns. I can't tell by the photos here, but you might ask your friend if there is also a gold plated vent hole liner. Both pan and vent hole were often done at the same time. If it's not a brass pan, I would say that you could easily date this blunderbuss to about the Third Quarter of the 18th Century.
Rick