Usually a Marinair or St. Ives 10468. This is the same input transformer used in the 1073, providing the initial "weight" and harmonic character.
Arthur stared at the original paper schematic. He needed to add a 10468 input transformer to step up the mic signal. He traced the pinout on the rear connector. Pin A to the transformer secondary. Pin E to the output. Pins L and M to the massive 24V power rail. Neve 1272 Schematic
The "magic" of the 1272 schematic lies in its specific transformers and the interaction between its discrete transistors. Usually a Marinair or St