So how do I take a chance on this?

[Electronic M]

If you do a quick search Zenith neck cones, RCA neck cones, and clone set cones were triangular in the roundy era. RCA/clones also briefly used a triangular cone on rectangular sets that was basically their roundy cone made more shallow. Both RCA and Zenith cones have a step to them. Zenith cones taper in to a smaller point. Both are distinctive from each other.