That's a so called "Lange s" (long s) which is no longer used in german writing.....
About the stamp... I can't tell if original, but there are many fakes out there, so i would be very very careful......
I know a little about stamps in general, and judging by the construction, the details, and the wear; I have no problem with this one and would say it is ok!
Always looking for Belgian Congo stuff!