From my observations.

OP uses two silver cards. One of them is ended on B4 while the other on 89.

B4 card holds letters in qwerty layout as on typical English keyboard while 89 card keeps letters according to their alphabetic order. 

Characters on silver cards feed message intended for encoding while characters on black card are relevant to digest.

It is very likely that SEED word coming for encoding  is split into two half and each of two silver card is design to encode its own half. Or those cards are used separately to encode even-numbered and uneven words.

But, it is still unclear for me why silver cards hold special characters and numbers. SEED words don't have any of them.

It is highly likely  that two characters (a few option for this) in digest serves as decoy as OP encrypts only first 4 letters from the SEED.