I had the same search on my end. The Lava is actually a newton, not a Rotring 600 ( https://unsharpen.com/pencil/rotring...afting-pencil/ ), which should help with your search.

The Rotring 600 was made in 0.5, 0.7, 0.9 and 2mm. The Newton pencil, in lava, was sold 0.5 and 0.7mm

This might help as well: https://unsharpen.com/rotring-600-vs...on-comparison/