The energy efficiency class of this bulb is G. On this light bulb you'll find a GR10q socket.
From Amazon
Osram CFL Square 28 W/827 4-PIN, GR10
LED Bulbs
Osram 28w CFL Square 2D Bulb GR10Q Very Warm White - 816951
Big on Electricals
Osram 28w CFL Square 2D Bulb GR10Q Very Warm White - 816951