I noticed this holster first on several Texas Rangers at the TTPOA Conference and went to track it down. Turns out it's custom kydex holster manufactured by DSG Arms, a Texas based tactical retailer that also manufactures holsters, belts, rifle magazines and other pieces of kit. The Alpha Holster appears to have good retention (I watched one of the DSG Arms guys turn his upside down and shake it to demonstrate that it wouldn't fall free) and rides close enough to the body that it doesn't print noticeably beneath an untucked BDU style blouse. (This doesn't apply to the puffed up guys who wear their shirt a size too small and sew elastic into the cuffs so you can roll your sleeves high and tight way up on yours.)
The Alpha can be adjusted for cant and height and can be converted to IWB carry with their J-hook (requires an additional if inexpensive purchase). They're available for most breeds and styles of holster, with and without lights. A lot of the Texas Department of Safety SWAT Ranger Division wear them, as do some of the DPS Aviation Unit, ALERRT instructors, Harris County Sheriffs Office, TTPOA staff members and numerous other police agencies of the Lone Star State.
I wasn't able to get one out on the range or spend any time with it, so I can't give you a review, but I did make it a point to ask a couple of the Rangers what they thought about it. Two spoke very highly of it and preferred the Alpha as their holster of choice. One was not wearing an Alpha and apparently didn't own one (leading me to believe it's not standard issue or mandatory, though I don't know that for a fact) and another in plain clothes said he wore one when he was geared up by preference, but not off duty.