CT330 Thicknesser blade change problem

I have just acquired a second hand CT330 thickness and wanted to check the condition of the blades. The instructions on how to change them are written on the steel cover over the blades and just to be sure, I watched a YouTube video on how to change them. However, on removing the cover, the cutting head looks nothing like the one in the video and following the instructions doesn’t result in the head locking to remove the blades. Here’s what I see when I remove the cover (excuse the superficial rust, it was bought from someone who kept it in a drafty damp shed.)

Can anyone suggest how I go about removing the blades from this and indeed which blades would be a suitable replacement?


Here’s the blade rotated a bit