I realised it must be Helvetica because of the "R". I reasoned thus: I am more familiar with Arial than Helvetica, and I'm sure I would have seen that "R" before if I were spending a lot of time with it because it's just so ugly, so since I didn't recognise it, it must be Helvetica. And I was right.

It's funny, because I universally prefer Helvetica over Arial otherwise.