Hi Pal,
BUT000 table stores General data of the Business partner. Entries in this table can be easily influenced/changed by using Maintain Business partner transaction. External BP no. that you see in BUT000 table is the value that is maintained under identification tab of Maintain Business partner transaction.
Whereas, /sapsll/pntbp is a table maintained automatically by the system while transfer/replication of data.
So, when we refer to mapping of BP number with external number we should always consider using /sapsll/pntbp table. External number that you see in BUT000 table can be easily changed in Maintain Business partner transaction. You can try this using transaction code BP under Identification tab.
Regards
Dhilipan
