Jim, thank you for your thorough comments. I have the Furusiyya Foundation book and it is a great reference. It shows a nimcha with a hilt, covered by turtle shell and silver (#41), and then mentions the 4 similar swords captured in 1732 and now in the Real Armeria. This info is again repeated in the recent Met book to which Kwiatek contributed translations. However, I have reasons to believe these turtle shell hilts appeared a century or so earlier, because a fine example now in the Rijksmuseum was captured by Michiel de Ruyter in 1655:
https://www.rijksmuseum.nl/en/collection/NG-NM-10412
In the Malta Armory, among the many nimchas there is one with a D-ring, but it is there nonetheless. These swords were trophies of the knights of St. John, captured in naval battles in the Mediterranean, in a period from the 16th to the late 18th centuries (the order seized to exist after that courtesy of Napoleon). So there are most certainly Maghrebi nimchas with this feature. The lovely example in the Hofburg (also shown in Claude's great book) also has a D-ring and since it is probably connected to the Habsburgs, it likely came from the Mediterranean too (and it is decorated in an Ottoman style).
When it comes to guard and quillon style, I wonder if the approach of trying to differentiate on a purely regional basis is flawed as some variations may be chronological in character.
Regards,
Teodor