I think the largest monitor Bally was using in Xenophobe's era was a 19". All of their late 80's games I can think of used 19's.
The Defender is distinctive... Shallow control panel, cut in at the base, diagonal slice out of the back. When you look at cabs long enough, you start to recognize profiles, and Defender is an easy one to pick out.
Centuri was the manufacturer of Vanguard. They used that same style cab for other games... Phoenix, Pleiads, Time Pilot and others all have that distinctive fat, squared off look at the top.