Happ Controls:

http://na.suzohapp.com/amusement_products/security/30-1627-00

A-1 Security Bars makes bars for all types of coin operated equipment, including pinball machines and arcade video games:

http://www.a1securitybars.com/

http://www.amusementdistributors.com/?main_page=index&manufacturers_id=73

http://www.twistedquarter.com/index.php?main_page=index&cPath=155_162

For an alarm system.

Yes, many operators did this. They used a nice loud portable alarm system that ran on a 12 volt lantern battery. It would go off as soon as the coin door was opened. Hopefully to catch or deter a crook. When the route money collector came through to collect the money out of the machine, he would use his alarm system key in that lock to shut off the alarm system while the collection was taking place. When the collector was ready to leave, he'd turn it back on again. The alarm system was portable and sat inside the machine in a convenient place. All the person installing needed to do was drill a hole for the alarm on-off switch.

When it came to to remove the alarm system, the operator simply filled the hole for the lock with a large carriage bolt and no one was the wiser. Back in the day, Wico and other big amusement parts suppliers sold these portable alarm systems.

Nope. That looks like it keeps tension on the flipper opto actuator. Those actuators lose tension over time and that looks like an operator quick fix.

That air horn was also an option for Rowe jukeboxes from models MM3 (1968) and up through CD100 machines. Once triggered from a break-in attempt, that air horn would not stop until the can was emptied (it was very loud too).