Originally Posted by
cyccommute
I think you are incorrect. The “lever boss cover” (part 6 in the picture) comes with the new shifters. The shifter should just bolt onto it. The boss cover may even still be on the cable guide side as the guide needs to sit flat on the frame as well.
The "lever boss cover" with the concave bottom (p/n
643 3810-1) comes with Shimano downtube shifters. It fits Campagnolo-style shift lever bosses. The other (p/n 643 4300) has a flat bottom is/was available to fit bikes with a corresponding shift lever boss such as Vitus or Cannondale.
Shimano has kept these parts the same for decades. This PDF for Shimano 600 shifters illustrates the different "lever boss covers" nicely:
https://si.shimano.com/pdfs/ev/EV-SL-6208-0735G.pdf