How to open this SeacSub 2nd stage cover?

[Aralox]

There are 4 slots along the front edge of the cover, with a little black plastic tab from the body in each. I've attached a picture of the right-side tab when the cover is not rotated, and a picture of the left-side tab (clearer) when the cover is rotated slightly clockwise, upon which it gets stuck.
Do I need some sort of special tool to open this?

Thank you

 

[Killerflyingbugs]

push the tabs with a flat head screwdriver and be careful.

 

[Aralox]

Thanks for the reply, but actually that wasn't the way to do it. I tried pushing the tabs with the screwdriver, but noticed that someone else had already tried this and had flaked up the plastic in one of the slots.
To open it, I had to
- soak the reg in some warm soapy water
- pull out the exhaust tee (is that what they are called?), which was preventing the cover from fully unscrewing
- then unscrew the cover clockwise slightly (about 1/12), and pull it out

Pictures: