Ask them to identify the complete missing kit against your agreed order and receiving check, then name the proposed supply arrangement. A reply about the fitted pendant cable doesn't address the additional one. Nor does a future shipment exist merely because the phrase keeps appearing.
Then ask for that as an explicit partial-supply option. Clara's suggestion can be useful without turning one cable into a complete kit, provided the outstanding contents and any carriage are stated rather than inferred.
Keep it there in the purchasing note. Availability being checked is an open enquiry, not stock or a delivery date. The partial option is defined now; the seller still needs to answer it.