Originally Posted by speedbrake

The last 2 digits of the Boeing designations is a customer code. i.e. 31-TWA, 22-UAL, 23-AAL, 32-DAL and many many more. That is why UA aircraft will often be shown as a B757-222, B767-222, B767-322, B747-422 etc. You can tell when a carrier has acquired a second hand aircraft or another carrier's delievery slot by comparing the last 2 digits.