there are two varieties of "Speck" which also exist in the language on this side of the pond:
fatback ("Rückenspeck" - Rücken=back)
pork belly ("Bauchspeck" - Bauch=stomach area aka belly)
a third variant "Schinkenspeck" is cut from the leg/ham - most used as a "cold cut" for breakfast. it's not "ham" one would want to use for a sandwich - it's a cut/cure/slice one drapes on a unsalted buttered fresh Broetchen half . . . toothpicks at breakfast +20 required.....
here's the mega-issue: every German town has (multiple) butchers who prepare their stuff in their own manner. in USA we have 2-3 pork houses and it is "all made the same." methods for Speck include salting, drying, curing, brining, smoking. along with dozens of spicing options.
for people with European exposure, the problem becomes not only is it "authentic" but is it the flavor of what the exposed considers "authentic"
I have never found any Speck in USA from any domestic source that matches my Swabia or Bayern experiences. so, basically if making you own - do it like you find it yummy and worry not about what it's "supposed" to taste like.
Pork belly is what is used to make bacon, so if you've ever made homemade bacon, you've smoked pork belly. We've done that several times. In fact, we may be making some soon, as I need to pull a slab of pork belly out of the freezer for Asian pork belly buns. Depending on how big it is, some may get made into speck or bacon, we'll see.
I believe fat back is what is used to make salt pork, at least good quality salt pork. I need to order some fat back from our heritage pig source, central Florida, because I want to make Lardo, just need to get the marble box made that I need.