The families of 10 workmen murdered in the Kingsmills massacre are to be given a history of the weapons used to kill their loved ones.
The chronological list is being provided by the PSNI.
It details where and when any guns fired at Kingsmills were used before and after it.
Some of the weapons were not recovered for decades and are linked to long lists of attacks.
Ten Protestant workmen were killed at Kingsmills when their minibus was ambushed in the south Armagh village in January 1976.
The men were ordered out of the vehicle before being shot. One man - Alan Black - survived despite being shot 18 times.
Thursday's development came at a preliminary hearing, paving the way for the inquest into the 10 deaths to restart next month.
It had stalled while the police investigated some new evidence around a partial palm print found on a van believed to have been used by the IRA gang which carried out the attack in January 1976.
The coroner, meanwhile, has told the families and their legal teams that he will not consider the inquest to be complete until he is satisfied it has been given all relevant material held by the Garda (Republic of Ireland police).
The families have previously expressed their disappointment that only a small amount of relevant information, much of it newspaper cuttings already in the public domain, has been provided by the Irish government.
Some of the families have said they will stop co-operating with the inquest unless all relevant material is made available.
On Thursday, the coroner revealed that he had written directly to the Garda but that he had yet to receive a reply.
The preliminary hearing will reconvene next week to consider Public Interest Immunity rulings relating to some documentation, and the inquest itself is scheduled to restart on 15 May.