Using the 1st triangle pcs (alternating single notches), assemble the first triangle

First pc of the 2nd triangle twisting on

First pc of 2nd triangle is now on the 'base' triangle.

Second pc of the 2nd triangle twisting on

Second pc of the 2nd triangle is now on the 'base' triangle.

Third pc of the 2nd triangle twisting on.

Third pc of the 2nd triangle is almost on 'base' triangle.

Closeup of the twisting action.

The entrance edges to the notches (on the lower side) were beveled slightly to smoothly initiate the locking maneuver.