What is this module?

Looks like the Vreg cable is separate from that unused 5-pin ECU connector and they just run the two in the same split-loom. They are probably using a different spark module, perhaps MSD or GM HEI, maybe a Pertronix Ignitor under the distributor cap or even an after-market distributor. The security switch is likely in the cabin, in reach of the driver. If a momentary push-button, then likely on the starter wire (yell). If an on-off switch, then likely on the IGN wire (dk blu or grn, forget).