Doing this from memory...
The 89' Ban was a Import Ban and concerned HK91s, AUGs, FALs, etc. Foreign made rifles had to become "sporter" models to be legally imported. These were the thumbhole stock SR9s, USRs, etc.
Around this time Colt adopted a sporter model known as the Blue lable. Pre89s are Green label, flash hider, bayo lug, etc. Blue labels have a flash hider (non removable I think), were available as a collapsable but neither had a bayo lug.
Post 94s banned assault rifle config, NO flash, collapsible or bayo lug on a pistol gripped weapon.
Now here is the rub, even if you have a preban receiver (Green or Blue Lable) if it was NOT in a assault rifle config, the ATF has declared it illegal to assemble it as one.
It sounds as if you have a blue lable. With the pistol grip AND flash hider it is already a preban assault rifle so a collapsible is legal.