I found this photo dating back to the 1980s on Pinterest and it's pretty interesting. Usually, there's a bottom set of horns but there's a skirt at the bottom of the 15v2T(?). Also, there's a set of batteries mounted on the bottom of the siren so it could be a 15V2T-B. But I thought Sentry didn't make those until the 2000s.
Orignal source: https://www.pinterest.com/pin/626563366889867879/