Abu 506 help

Help! I want to put new line on my ancient Abu 506 but I can't get the spool off. I know it's not difficult as I've done it loads of times before - I thought it just unscrewed - I've pressed the black button which pushes the side pin in, tried both clockwise and anti-clockwise but nothing is happening. I don't want to force it and risk breaking it. What am I doing wrong? I hope somebody out there will be able to help!

Hold the reel with spool facing away from you and twist left to unscrew the rotating cup - there's usually a sticker on the winding cup; yours must be missing, I guess.

If the reel's been unused for a while it may be quite stiff, but it should give under pressure.