Note to self:

... don't sow if it's too hot and sunny, lithops don't like it very much.
I was already getting worried as the Green Horn and Ventergreen seeds took their time germinating. But after a couple of rainy days there they came. I'm sure they would germinate even better in the fall: these were harvested in February and might still be a bit too fresh. 
L. lesliei v. hornii 'Green Horn'
L. lesliei v. venteri 'Ventergreen'
These were a product of selfing and I actually didn't expect anything to germinate. We'll see how they'll develop.
Thanks for your comment