In terms of the rollbar, I remember seeing photos of some 1983 TxDPS units that had them. I don't know about latter year cars.
I kinda remember that 93 TxDPS. The new owner may be on one of the social media sites now.
The full story behind that white 89 car is a mystery. It does say TEXAS on the Buck Tag and it is at the beginning of a batch of VINs with confirmed TxDPS cars. It may or may not have been used by TxDPS. Or perhaps another agency in Texas? I would want to put my eyes on it before shelling out the kind of money the seller wants. We are seeing several instances where SSP Mustangs are selling for >$20K yet the cars are incorrectly restored (some are not even "restored" but are merely freshened up). So buyer beware.
Goof luck on your search, Phil!