Sorry for the delay, the ball didn't quite work so I need to wait for a proper back screw to open it, it reads T 25, anyone has an idea whether it is UG in house or outsourced? Serial no. Is exactly 2,600,500 and reference no. 825604/01. It also has a screw down crown with inner spring like a male sub, sorry for the iPad pictures, I'm not really a camera aficionado. Saving this for my 2 year old girl in the future