Does anyone know the specs of the screw of part #7? Or where to get it in US?
It unscrewed and got lost. I think I might find something similar not bike-specific.
Your local bike shop should have access to either
Or your final option is to take the one out of the other shifter, and measure it. You need the width, length of thread, head shape and dimensions, tool interface, and thread pitch. With that you should be able to find the right thing at a specialist fastener supplier, or maybe at a hardware shop (unlikely)
You may need to buy ten or a hundred at a specialist, but hey, that's spares, and may still be cheaper than a one-off via Shimano spares.
I found Shimano doesn't have spare parts for the shifters, at least not that I or my LBS has. They usually have broken or spares that they take parts from. One of my BIGGEST gripes with shimano. Campy you can do a total overhaul on their shifters with pretty much all new parts. Typically the springs are the issue though. Not easy to do but possible. With shimano, just replace or get it replaced through warranty.