Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Diamante citron
Genoa was known to supply citron for the Jews since the times of the Tosafists, along with surrounding municipalities Sanremo, Bordighera, and the rest of Liguria. The city is located in the region of Liguria, which itself has a long history of citron cultivation, thanks to the massive mountain chain (Apennines) which protects it from turbulent winds.