Tiny screw for dura ace shifter??

Go to the shimaNO website.

Then to Tech Docs.

Find screw on exploded view drawing: http://si.shimano.com/pdfs/ev/EV-ST-R9100-4057.pdf

Detail 11 is screw plus cover. Note left and right hand part numbers.

Call shimaNO with credit card in hand.

Of course ...
[INDENT]
You can do as CAMPYBOB suggests and contact Shimano (who are generally VERY helpful) ...

OR, because there is a 99.99% probability it is a metric threaded bolt you can probably just go to a Home Depot and buy a small packet of replacement bolts (you generally can't just buy one).

[INDENT][INDENT]If you don't know the size, then beginning with your set of Allen Wrenches, insert the smallest (e.g., 1mm) into the threaded socket ...
[INDENT][INDENT]
Continue with progressively large ones until you find the one that you CANNOT insert ...[/INDENT][/INDENT]

There is a 90+% probability that the wrench which doesn't fit is the size of the threaded bolt that fell out ...

[INDENT][INDENT]Now, you just need to decide how long a bolt you want ... [/INDENT][/INDENT]

Then, go to a HOME DEPOT and ask the individual at customer service where their drawers with the small metric bolts are -- you will probably get a packet of 2-to-4 for about $0.50 (50¢) depending on the thread size & length.
[/INDENT][/INDENT]
If you are daring, then you can simply remove the identical bolt from your other shifter & tape it to a file card with a broad piece of clear packing tape ("unattended" small pieces tend to wander off on their own) & bring THAT bolt into the store so you won't be guessing as to the size.

[/INDENT]