:As Jpatokal said, "Kuwa"(クハ,クワ,クヮ,クァ) in first version of Hepburn's dictionary was "kwa" in second version or later (『和英語林集成』各版ローマ字対照表 by Meiji Gakuin University), and "kwa" was abolished in Hyōjun-shiki Rōmaji (standard style) in 1908 (標準式ローマ字綴り方 by Daijisen). 
 :Well..."kwa" for other languages seems to be in British standard for Japanese romanization (BS 4812:1972) that is one of the (revised) Hepburn romanization family. "Gwa"(グハ,グワ,グヮ,グァ) ("guwa" in first version) is so. Do they are needed in "For extend katakana" section? Umm... 
 :Incidentally, "クワ" and "グワ" is left in some Japanese dialect such as Unpaku direct (拗音 by Daijisen, 雲伯方言 by ja:Wikipedia).   

 == modified Hepburn == 
 Is thw following explanation right in "Variants of Hepburn romanization" section? 
 :It has been adopted by some major dictionaries (e.g. the Pocket Kenkyusha Japanese Dictionary published by Oxford University Press). 
 Kenkyusha's New Japanese-English Dictionary follows romanization called "modified Hepburn", but "modified Hepburn" means what is called "revised Hepburn" family in this case. Does Pocket Kenkyusha Japanese Dictionary follow "modified Hepburn" style such as "Tookyoo"? And, what is other major dictionary adopting "modified Hepburn"?