Yes, I did some more digging on this after my post. I found one on EBAY and purchased it, looks just like the one on your picture. Has all the original hardware and wiring. It was cheaper than I could recondition my original ON-GUARD siren, plus benefits of new hardware and the actual instructions. The only difference was the end of the unit, mine had a baffle rather than the chrome horn shape. When I installed it yesterday into the original position/same holes below the voltage regulator, I replaced the new horn end with my original baffle. The rest of the unit was exactly the same, in fact my baffle attached onto the base in the same holes and screws. The new chrome horn wouldn't fit without touching the radiator because it was too long.
I have the extra door jamb switches, so it should be easy to rewire. Project still underway. The keyhole is a little different, and the keys are a newer style than my original, but the original keyhole had the tabs broken off the back.
If anyone is interested I can post pics when I finish installing it.
Thanks - Andy