Kenny, you can find the specs in a post which were stated 3 1/2 years ago here the forum by Mr. Duffey when he was looking for a valuation of the gun.
http://parkerguns.org/forums/showthread.php?t=12554
You can see pics here:
http://parkerguns.org/forums/album.php?albumid=530

Thanks Wild Skies......This would have been a non catalog item that would have needed all the barrels to have the same S.N. Other than that all the components were available to assemble. I am surprised that more sets were not sold with all the barrel options and chokes like this set. The added cost $$$ may have played a big part in it or the venders did not push the idea and the buying public did not know guns could be special ordered outside the norms. thanks all SXS Ohio

Unique Parker Specs?
 

Here are the "specs" I have on the 20GA 3 barrel set:

20 GA - 3 barrell set
2 barrels are 26" and One is 28"
28" barrel is stamped Mod & Full and 2 3/4 & 3"
Both 26" barrels are 2 3/4 only
One is stamped IMP & MOD
The other says Q1 & Q2
All 3 have same serial#: 20-04440
Purchased 6/24/88 from Paul Jaeger, Inc.
Grand Junction Tennessee
Have original sales receipt, Parker Repro booklet, etc.

Tried to post specs but don't see them in this thread?? I'm a 66yo TOTAL TECHNO-IDIOT so may have done something wrong. In reference to the smart asses who were making fun of my use of "unique" I ASSUMED being the "only" one of it's kind at purchase and possibly the FIRST if more were produced later qualified it as "unique" or "one-of-a-kind?" Excuse me if I'm stupid or naive. :-(

OOPS! Guess it did post. :-) THANKS Wild Skies for the LINKS to my previous posts. Plan to advertise for sale but apparently have to join first. Will try to do that soon.

Three Barrel Set
 

I don't recall ever seeing a three barrel set with all barrels being 20GA. And I also believe your father owning the gun certainly adds to the value. He was a very good writer and I enjoyed reading his works and getting his insight. I would love to own one of his guns.