This is the current version of the Model 510. It's the same razor as the Model 500 HERE with a longer handle.
It's designed as a two piece. The adjuster knob is held in with a compression ring and pulls out very easily. They have kept the same head and added a longer handle and made the shaft of the twist knob longer to account for the longer handle.

How does it shave?
All Merkur Progresses are great everyday shavers. My preference is shorter handles but if your's is a longer handle this would suit you.
The go from pretty mild to middle medium at 6. They then can be overclocked on the fly if more is needed.