Looks like the serial number checks out. It has the small m too. Jupiters have a capital M. I have a Jupiter from the 50s and it is a nice lens.
If you like it, use it. Doesn't really matter if it is fake or not! I really love the Jupiter I have. It isn't as good as the post war Sonnar I used to have in Contax mount (miss that one!) but a wonderful lens.
I hope you would be so kind as to post some images from it. I love Sonnars. Might end up picking one up myself. A Zeiss Sonnar would look good with an M3 attached...