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Esslinger sells one just like the one from Bergeon (Swiss Quality brand), but for about $70
I've looked and looked on the site and can't find it - do you have a direct link?
The scratches come from the end links's sharp edges, not from putting on or taking off springbars.
Coincidentally, yesterday I just put the 677 ends on my SM300. They seemed to be too big at first when I trial-fitted them and my initial thought was also to file them down. Turned out just some wiggling at the right position and angle took care of it.
Could you please post a pic with you SM300 and the 677 end links?
Here you go. Someone compare thhe 677 vs 633 on a Speedmaster Pro here: http://forums.watchuseek.com/f20/633-versus-677-end-links-1025859.html.
Hope it helps.