bleeding radiator

I have a small double radiator that is only heating on front panel after I had to drain and refill the system. it has a normal key opened bleed valve on the front panel but a strange looking object, like a brass nut with a hole on each face and what looks like a button on the end, in place of the bleed valve on the back panel.....

If this doesn't make any sense I can post an image...

Don't want to brake it buy pulling, pushing, or twisting to hard as nothing seems to move!

should be same on both panels. perhaps someone has 'altered' rad in past. pic please?

Sorry I didn't realise that you had to link to a pic rather than just post one
i have taken a photo but don't have any were to post it