By 'liter' do you mean 21j?
One of them is 21j (with the blue protective wrapping), the other is asian/clone ETA 2836.
I believe that these versions from Robert are from the Milgauss factory. The rehaut engraving appears different to (more accurate than) the 2836 on Josh's site, so I'm not sure if they are they same.
Either way, Robert's prices are the cheapest you will find, but he operates under different rules to other dealers.
If you are considering buying one, I would warn away from the '3131' movement as it is not very good quality, going from other posts I have read. Stick to clone 2836 if you want the correct sweep.