[Erik van Straten]

Never heard of a lens protector with a lens in it. I think it's a close focus lens. Presumably when this object is screwed into the lens and the lens itself is set at infinity, the focus will be at 50cm. But this is only speculation.
Leica conversions allways carry their original serial numbers. This was done for reasons of tax. When tax was paid for a camera with a certain number, tax couldn't be charged -- after the conversion -- for a second time.
I cannot give an approximate value. Much depends on the inner condition of the camera, such as the shutter cutains and the condition of the rangefinder. When the lens is in really good condition, it's probably worth more than the camera.
Re-reading your post, I think you do not know how the rangefinder works. Well, on the back of the camera are two peep-holes. The one on the right is the viewfinder. When you look trough it, you will see the image that will be recorded on film. The peep-hole on the left, however, is the rangefinder, needed to focus the lens. When you look through that one, you will see two superimposted images. When you turn the distance-scale of the lens, you will notice that those two superimposted images will move in relation to each other. When you point the rangefinder at an object and you turn the distance-scale of the lens, at one moment the two superimposted images of the chosen object will be co-incident. Then the focus is automatically set for the chosen object.