FWIW: it's not "unity gain", unless you turn the gain-knobs to the {UNITY} mark.
By the simplicity, it is probably a single transistor inverting amplifier. For more than a few signal paths, sanity recommends non-inverting.
The inverting mixer with over-unity internal gain is susceptible to hiss troubles.
I think in this day of 19 cent dual opamps, you can do better from a clean sheet. The woods (well, innernet) is full of mixer pages. Jack AMZ Orman, R.G. Geofex Keene, Rod ESP Elliott are some no-BS sources.