The brief answer is that, aside from Mercury, early stereo mixing boards had treble and bass, boost and cut pots that, and judging from the hundreds of engineers recording sheets I have seen from the 50s and 60s, were often used to adjust the sound of an individual microphone prior to printing the mix to tape.
As for the microphones themselves, the condensors commonly used then had a nice treble boost from ca. 9K upwards - this added a nice tingle to the sound. But this boost was most common when the mikes were used 'on-axis', and was less when the sound was coming 'off-axis'
Insofar as treble boosting at the cutting stage was concerned, LP cutters had limiters and other black boxes that were used to
transfer from tape to lacquer. The most common modification at that stage was blending bass to mono below about 150Hz,and adding diameter EQ (a treble boost) to overcome the pinch effect as the groove reached the label.