I agree it should not be there but ... It is not your on-line log-in account number, it's your actual account number.
Between registered IP addresses and the security tokens I think it's pretty useless to a hacker or other thief going through your trash. But IB should get rid of it. In general the account name and the info on the statement identifying the account and the name of the advisor should be enough for an advisee to figure out what's going on.
Again I agree and IB should drop the number from the envelope.
Jack