Siemens ecu number difference?

I need to buy second hand ECU for my car Ford Fiesta 1.4 tdci 2002. My car ecu has number 2S6A-12A650-BG SID802 1BSB
But here in local shop I found the same ecu but 1BSB is 1BSF, i want to ask what is the difference between BSB and BSF, if it fit in my car? What is the meaning of this B S B and B S F?

Hi this s like asking what is the difference between an xbox and Ps4. Yes it is very much the same issue here.
it needs to be an exact match other wise it is not going to work. The ECU or engine control unit is a computer like any computer. So trying to put the wrong one in a engine bay is not going to cure the problems. In fact it generates more problems. So apart from the very expensive 'Ford replacement part there's no another that will do the job.