Regarding the silencers for the Model M carbs, they do show up on ebay from time to time, so keep an eye out there. You really need one , as the choke butterfly is part of the silencer. Be aware there are three sizes- one size for each size carb.
Regarding the S/SR question. Yup, the SR's have a vacuum operated accelerator pump built into the bottom of the float bowl but most folks I'm aware of block off the pump by rotating the bottom cover position. Nothing wrong with an S carb in this day and age.
Jim- are you aware of a source for gaskets? Is Barry still offering them, or is there a new source?