Congrats on finding an 88 TxDPS SSP. The unit Number I have never seen as the tag/plate.
Here is some restoration info.
http://www.specialservicemustang.net/DPS.htm
Here are some pictures.
http://www.specialservicemustang.net/Gallery/TX.htm
You have already found the unit number so you are far ahaed the rest... Was this car an automatic? Seems to me that the Houst DSO is where the Automatic DPS cars came from. I have seen another one that was still in service with another agency in Texas and the DSO was Houston. Very likely this car was one that was bought by another agency when it came out of duty with DPS.
Good luck on finding all the info you need... Tom Mallette, Tony Sharp, and Roman Collins (there are a few others but these are in and around Houston) are the true Texas DPS restoration guys as they currenlty own restored Texas cars. Mine is still a roller as well as missing the front sheet metal.