the us made g500 was basically a g250(later called the V), the Italy made one was basically a s600. I had a few of the US made g500 and loved them but if I remember they killed them after the recall and went with the Italian so they could show a total redesign. Recalls suck.
I know the black purge covered G500s were made in the US and the gray purge covered G500s were made in Italy. I think, and have had others agree, that the US made G500s were better than the Italian version, and better than the S600, but I think that had more to do with tooling and minor changes than where it was made.
Tabata made the Mk 5, so their relationship goes back a long way. I've never had any solid information on what was made where or how many outside sources Scubapro has used, but Tabata has been in the mix from the Mk 5 to at least the Mk 16 and their production facilities are state of the art.
The earliest (mid 60's) Scubapro second stages look exactly like Beuchat second stages of the same era - and if you see the first Jet Fins, you'll see the Beuchat name on them.