This is a lower gain 12AX7 tube type.   Stevie Ray Vaughn used these to change his front end distortion to back end distortion ratio to have more of the touch sensitive output tube distortion.

These test as per 5751 spec with the proper transconductance and current output.

The slightly lower gain quiets many amps down and these are a very smooth front end tube but are also quite amazing in the phase inverter position.

In the Fender Hot Rod series these are really the rage these days with many of GT's folks that use these amps.