Hello everybody:
It seems that the interface cable for a 54120A/B system is very simple
to clone. All connections are straight thru and it can be built in
about an hour.
You can get all the parts from digikey
MB50H-ND is the flat shielded cable (5 feet)
MDB50K-ND (two) are the connectors (50 pins Delta connector)
MDBF50-ND (two) are the backshells...plastic no metal version
MDSF50-ND (two) are the strain relief brackets.
Some extra copper tape to shield the connectors and it is ready to
play.
Cheaper (about $92 with shipping) than paying the $160 from a dealer.