Scheider Kreuznac Symmar/ 5.6 /240 white between the Kreuznach and Symmar is the word sinar in green and opposite that also in green is 12/420.
Okay, that sounds like a Schneider Kreuznach Symmar, not a Symmar-S. The Symmar-S is a later lens. I have and regularly use a Symmar-S 240/5.6 and while it can indeed be used with e.g. just the front group, I don't think it's officially intended that way and the aperture scale is also not marked for it. I have 'in a pinch' used my Symmar-S without the rear group on occasion to get 8x10" coverage.
Does this mean it is a 240mm for 8 * 10 inch use and 420 mm for 4 *5 inch use ?
Not sure how familiar you are with these convertible lenses. The way they work is that you normally use the lens with both the front and rear group installed; i.e. the part at the front of the lens board as well as the part at the back of it that sticks into the camera. In that case it's a 240mm/5.6 lens in your case. If you remove the rear part, the remaining front group is a longer focal length, 420mm in this case, and since the front element and aperture don't change in physical size, the numerical aperture (as defined as the focal length divided by physical aperture) changes, so in this case it becomes an f/12 lens.
The focal length is regardless of what size film you put behind it. What does change, is coverage due to the longer vs. shorter focal length and thus longer vs. shorter distance between the lens and the film plane. As a 240mm lens, it will cover 5x7" (IIRC) at infinity focus. As a 420mm lens, the lens is placed at a larger distance from the film plane to get the same focus, and thus, the projected image is larger, and the lens will cover 8x10" (although if memory serves, only just).
