On Side B the suffix BXE is used. Don't get confused, it is Basic XL, OSS just used the suffix BXE for that disk.
According to Alfred from AtariAge, there is a problem with EXTEND: Wrong is that the MOVE $0570,$C4,4 statement doesn't work. $0570 is loaded with the proper values, but the cart command doesn't actually move the four bytes. If you deposit the proper values in $C4 and $C6 then the extension works. Therefore please use: MOVE $0570,$C4,-4 to install the extension. Thank you very much Alfred, we really appreciate your help.