Hi-tech forensic techniques not available at the time of the offence are being used in an effort to pin-point the genetic fingerprint of the senders of the letters through traces of saliva.
The findings will then be cross-referred with the police DNA database for a possible match that could identify some of those who sent the mail and may be important witnesses to the unsolved crime.
The move is part of a near two-year long 'reassessment' of the evidence by West Midlands Police anti-terror officers that is close to its conclusion but has so far failed to unearth any new lead.
Launched in June 2012, it has collated and preserved 18,500 items and commissioned an independent review of potential forensic evidence.
Chief Constable Chris Sims explained: "We do not want to leave any stone unturned and still have a few loose ends to tie up before the inquiry is concluded.
"These include trying to acquire DNA from envelopes which contained anonymous letters with possible information about the pub bombings."
Mr Sims recently met with relatives of two of the 21 people killed in the blasts that wrecked the Mulberry Bush and Tavern In The Town pubs in Birmingham city centre on November 21 1974 that also injured 182 victims.
Now he wants other people either bereaved or hurt by the blasts to have similar briefings on investigation. He said: "I have real sympathy for the emotion and agony endured by the families of those killed. I admire the dedication shown to keep the tragedy in the public eye. The bombings and those who died should never be forgotten.
"One of the most striking features of this case is the difference in the ways the families were treated in the 1970s compared with now.
"There were no Family Liaison Officers, no referrals and no Victims Support.
"I would like to offer families of those who died or people who were hurt on the night to be given as much information as possible and to answer what questions we can. We would be really pleased to give them a chance to ask any questions and update them as to what has happened. Hopefully this could give reassurance to hundreds of families."
Any of the families that wishes to take up the offer should ring 101.