The better question is, why do you feel you need to remove all this accumulation, unless you never plan to shoot the gun again?
Life is short. Cleaning, and recleaning, and recleaning, and then once again cleaning that area is futile. The accumulation is normal, especially with .22s. I have a 617, and lots of others. I keep them clean, but do not remove every trace of lead and carbon from that specific area every time I shoot each one.
When there is a significant accumulation, and when I have nothing more important to do, I carefully chip it off with the appropriate size screwdriver from an inexpensive Chinese set of six, available everywhere. There is a size to get into every spot in that area. Chip a little, swab with Hoppe's #9, chip some more, swab some more. But only if you have lots of time to waste, or only when there is a big buildup...after hundreds of rounds.
But I must admit that cleaning this stuff out of the compensator on my early Model 41 is a major pain.
If you put a few microscopic scratches under the top strap, who cares? the Correct Police are unlikely to catch you, and if you slip and make a more visible scratch, cold blue will fix it...until it is again covered by more lead and carbon.
These are wonderful guns, but they were made to be fired, rather than excessively fussed over. And I especially like the stainless guns because you can clean this area quite thoroughly without fear of scratching some bluing. I really enjoy my 617.