What is the brand or type of this track lighting fixture?
With the offset it’s likely a 2-circuit track.

I can’t tell from the picture, the Wikipedia article suggests measuring 7/8” vs 1” might yield a clue. (but the article is inconclusive and doesn’t list all types). 

Most electrical supply houses have a box of short track pieces and track parts under the front counter for comparison and matching. 

OP has determined he has Juno (type J) 2-circuit track this info, which was originally supplied in comments.