Sorry for the silly question, but I recently found a second-hand 8x10 Paterson proofer with 120 and 35mm glass and I can't find a manual online. I would like to swap the 120 glass (currently installed) for the 35mm. I assume I need do something with the 'little black hinges' but am worried if I use force I'll break it. I am missing something really obvious here... perhaps the glass can't be exchanged at all? Any help appreciated. Thanks.