Student or Learner
500 pcs of CKD1612 had / were postponed to June
"[Last week] 500 pcs of [the] CKD1612 [order] were postponed to June"
"500 pcs of CKD1612 have [now] been postponed to June"
"500 pcs of CKD1612 had been postponed to June, but can now be supplied in May"
All these work for me, depending on the intent - "had" doesn't.
I might have also written "until" rather than "to"
Hope this helps