[TeMPOraL]

I used this one:

https://www.amazon.com/Lego-Gray-Brick-Separator-LEGO/dp/B00...

I wonder about the design change, with this plus-like protrusion at the top. Is it to enforce correct direction of operation when using the separator?

[Tomte]

It is to push out cross axles that are part of Lego Technics.

See for example set 10074-1.

[friendzis]

I guess that "plus-like protrusion" is designed to fit "tubes" on the high bricks, primary use being removing low bricks from high bricks. I guess one could use two separators side-by-side to firmly hold 4x2 brick, IIRC 2x2 had one "tube" and 4x2 had two "tubes".