Visually Erfa Rasants look rough and ready, no-frills razors. They operate the same way as the Apollo / Merkur Progress, with a spring loaded middle plate and a "bottom dial" twist knob.
To the best of my knowledge there are 6 Generations.
(Yes - I'm missing the sixth, so if you have one that you are willing to part with please contact me.)
They appear to alternate between changing the handle and then the head section.   The TopCaps all look similar and can interchange but there is slight change between the Gen 3 and Gen 4 versions. The Gen's 1, 2 and 3 have a slight recess on each side that expose the blade tabs, while the Gen 4 and 5 TopCaps completely cover the tabs.

The images below are in the order that I believe them to have been released. If you know this to be incorrect please contact me.