fluorescent lamp socket confusion

Hello members, Recently a fluorescent lamp failed and I decided to replace it - I do not remember how but I managed to remove the lamp. Now I have bought a new lamp (T8) to replace - however I have failed to understand how this lamp would install. The socket seemed linear, but the black lock inside would not rotate. The lamp pins would also not fit the centre (snap in type). Please see attached picture of the socket. Attachment 39148 (http://www.doityourself.com/forum/attachments/lighting-light-fixtures-ceiling-exhaust-fans/39148-fluorescent-lamp-socket-confusion-photo.jpg) Appreciate your help and advice.