The Cat Eye H-27 bracket fits the following Cat Eye lights:

HL-EL510
HL-EL500
HL-EL300
HL-EL210
HL-EL200
HL-EL110
HL-EL100
HL-NC300
HL-NC260/250
HL-1600
HL-1500
HL-500II
HL-500

It fits bars 22.2 to 25.4mm in diameter.
For 31.8mm bars, Cat Eye offers the H-31 bracket.