How to increase friction on a Shimano SIS+Friction rear shifter when it is in friction mode? I'm using some Dura Ace SL-7700 downtube shifters on my road bike. I bought them NOS.  The right (rear) shifter has the D-ring for switching from SIS to friction mode.  It's a bit strange that the user manual didn't include mention of the friction mode, and while the D-ring on the shifter itself does have the little triangle marker that points to "SIS" stamped on the lever, the lever doesn't have 'Friction" stamped a quarter-turn away like it does on my older 8-speed shifters with the same feature.  Yet, if I turn the D-ring a quarter-turn, just like with my 8-speed, I get friction mode.

However, I feel like the friction mode doesn't have enough friction.  The front (left) shifter has plenty of friction, and it is easily adjustable by turning the D-ring.  What do I do to increase friction on the rear (right) shifter if the D-ring is used to change between SIS and friction modes?
There should be a screw opposite the D-ring that you can tighten down to increase the friction on the shifter. On a shimano shifter a flat head driver should be used, but you can use something like a dime if you're in a pinch.