R5-BX1 (or Bell as it is also called) is a custom R5 unit ordered by Cha'rels Yagar, an influential Republic Judicial Force captain prior to the outbreak of the Clone Wars. R5-BX1 is the only R5 unit in existence with a transparent head. When Cha'rels Yagar joined the Rebel Alliance a year before the Battle of Yavin, R5-BX1 went with him.

*side note* R5-BX1 is a droid I came up with. It's designation is a reference the Bell X-1, which took part in the first manned supersonic flight. This is also why the droid has orange markings.
https://en.wikipedia.org/wiki/Bell_X-1