That round felt seal comes in the rebuild kits for all 8BA, Ford EBU and Holley 2100 "bugsprayer" type carbs.
The best way to install it I found is to press the felt seal into the recess in the bottom of hole for the accelerator pump actuating shaft. Then push your accel pump shaft into the hole in the main body from above while holding the felt seal in place with your thumb and poke the shaft through. The seal will stay in that recess at least initially.
I have them on my Ford EBU carbs. Prior to installing the accel pump shaft/assembly, I rub lubriplate all down that bore thru the main body that the accel pump shaft goes into. That lubricates the inside of the felt too once the shaft is installed.
For Holley 2100's and Holley 2110's, they don't have that bore at all. It's just a big cavity, so there is no reason for lubriplating the shaft.
Sal says there is supposed to be a thin washer that holds that felt in the recess. If so, it doesn't come with the rebuild kits and all three of my EBU's never had that washer.
Also, the instruction sheet that comes with the kits does not show a washer for that felt. Gonna look at the Ford shop manual and see if they show it. I run mine daily and will keep an eye on that felt and see if it works it's way out of the recess.