I found this circuit board from an 1980s old humidity controlled chamber. This definitely looks like a temperature regulator of some sort. It has a M logo which means made by Motorola. It has the labels E03G-3-9 and 8739. Does anyone have any info on what this part is?
It matches rectifier Case 175-03 or 253-03 used on SCR's with an obsolete P/N made in 1987 week 39 (8739). Appears to be rated for about 25A with 4 switches to serve as 2 half-bridges for 2 loads with variable phase or ON/OFF control. It takes 2 SCR's to make a Triac.