I went to Soundtracs' website. Defunct. I went to the usual manual archive sites. Nothing but the 24:8 version. I went to Reddit. I got five replies, all saying: "I have a PDF. I'll scan it tomorrow."
The scarcity of the is a perfect storm of unfortunate events: Soundtracs Topaz 12 4 Manual
MIC ──┬─ [GAIN] ── HPF ── INSERT ── EQ ──┐ LINE ─┘ ├── [FADER] ── PAN ── ROUTING ── GROUP OUT AUX 1-4 ──┘ I went to Soundtracs' website
For specific parts or deep technical manuals, community forums often point to TheSoundtracsGuy.com , a specialist for these legacy consoles. Key Specifications for the 12-4 community forums often point to TheSoundtracsGuy.com