looking for this special tool

[Youngwhisk]

Look for angled snap ring pliers.

 

[rackmonkey]

I think snap ring pliers are made for the round holed spanner nuts, aren't they? I use a pair of those to tighten/remove angle grinder lock nuts. The nuts the OP attached pictures of have slots.

But maybe they make slotted snap ring pliers. I've not come across any though.

 

[Gold]

I think snap ring pliers are made for the round holed spanner nuts, aren't they?

Snap ring pliers are made for circ clips . I’ve seen the nuts you are talking about. I’m not sure snap ring pliers were the intended tool. Although they work fine.
I
I keep all the blade removal tools that come with grinders and saws. They are all different. I keep them together like a key ring.

 

[rackmonkey]

I was just responding to the previous poster’s suggestion. If you click the link, it takes you to a pair of snap ring pliers.  So I was questioning why those were suggested (while leaving room that there might have been something I was missing)

 

[andre tchmil]

I'm Back with proper picture. !!
as you can see the screw thread is long so I think I need this Swisstool to open it properly.
I have a dozen of these meters x 4, it's worth to find the right tool.

 

[john12ax7]

Seems like you need to find a spanner of appropriate size

https://www.okw.com/en/Acc.-Combination-Knobs/A6300000.htm

 

[andre tchmil]

Seems like you need to find a spanner of appropriate size

https://www.okw.com/en/Acc.-Combination-Knobs/A6300000.htm

looking good and probably cheaper than the Swisstool but not in M3.
Maybe I DIY one myself from a small pipe.