What is the difference between SNAP-IDC-FO and SNAP-IDC-HDB?

What is the difference between SNAP-IDC-FO and SNAP-IDC-HDB? Can I replace each other? I have not found any information about SNAP-IDC-FO.

Can you tell me if the SNAP-IDC-FO has a printed R##### on the board somewhere, I cannot see one on the top, but it may be on the underside of it.

They are the same board.

R81021 is the part number of the PCB which is used on both products. Also, the date code is from 2005, which is when the last change was made.