Bally was so cheap, they probably bought factory "seconds" of regular truss head screws.

I've been using zinc truss head screws on the pinball machines I restored for Welby Bergum and they look perfectly fine.

On the machines I did for Welby, he took one of the original truss head screws to his local branch of Tacoma Screw Products (just a mile down the road from his house) and Tacoma Screw got him the exact replacements in zinc and for only 2 cents each in the 100 pack bags. It was special order and it took a month, but it was worth it. I think he bought 5 or 6 bags of them.

Tacoma Screw normally stocks the #6 by 1/2" long phillips truss head screws only in stainless steel (perfect for replacing the drive nails on side rails).

http://www.tacomascrew.com/s.nl/it.A/id.21694/.f

Here's a company that has them dirt cheap (2 cents each only sold in bags of 100) in nickel plated:

http://www.woodworkerexpress.com/Truss-Head-Screws-Hinge-Accessories-Truss-Head-Wood-Screws-Description-6-x-1-2-2-Phillips-Truss-Head-Wood-Screw.html

More in zinc:

http://ahturf.com/store/index.php?route=product/product&product_id=21510

Fastenal has some in zinc:

https://www.fastenal.com/products/details/0139246?r=~|categoryl1:%22600000%20Fasteners%22|~%20~|categoryl2:%22600051%20Screws%22|~%20~|categoryl3:%22600066%20Sheet%20Metal%20Screws%22|~%20~|sattr03:^Truss$|~%20~|sattr02:^Steel$|~%20~|sattr04:^Phillips$|~%20~|sattr05:^%229pound6%20-%2020%22$|~

https://www.fastenal.com/products/details/0142431?r=~|categoryl1:%22600000%20Fasteners%22|~%20~|categoryl2:%22600051%20Screws%22|~%20~|categoryl3:%22600066%20Sheet%20Metal%20Screws%22|~%20~|sattr03:^Truss$|~%20~|sattr02:^Steel$|~%20~|sattr04:^Phillips$|~%20~|sattr05:^%229pound6%20-%2020%22$|~

https://www.fastenal.com/products/details/31228?r=~|categoryl1:%22600000%20Fasteners%22|~%20~|categoryl2:%22600051%20Screws%22|~%20~|categoryl3:%22600066%20Sheet%20Metal%20Screws%22|~%20~|sattr03:^Truss$|~%20~|sattr02:^Steel$|~%20~|sattr04:^Phillips$|~%20~|sattr05:^%229pound6%20-%2018%22$|~

Pinball Life also sells the truss head screws in zinc:

http://www.pinballlife.com/index.php?p=product&id=1137

Yeh, what he said.