Plastic box approved for ceiling fan?

Hi;
I just re-installed a ceiling fan which had been installed about 10yrs ago, then removed and stored for about 7yrs.
The box that is in the ceiling is a plastic RAYCO, which does not look like a normal box. It has a large section in the middle which takes up a lot of the volume of the box, and this is secured by two large, recessed screws.
The box itself appears to be securely attached to the joist, but I did not go into the attic to check, since there was already a fan there.

I had always thought that only metal boxes were approved for a ceiling fan, so I am a little concerned. That said, there is an identical fan in another room that was installed by the same electrician at the same time as this one, and I assume (I did not take the fan down to check) that it is also a plastic box. That fan has been up and running for ten years with no problems at all.

Thanks for your advice/opinion.

FW