The schematic that came with my AL-1500 and the schematics I find on the internet show +12V being used to throw the antenna relay.
All the AL-1500's (and I think AL-1200's) I've ever met in real life have a modified meter board to use a voltage doubler (caps and diodes), to generate a new +24V supply for the antenna relay. I'm sure there's a different part number for the 24V antenna relay board too. Maybe a couple of these were user mods of early amps, but I'm sure the vast majority came out of the factory to use +24V.
Above voltages are "nominal" really they are substantially higher at least when the relay isn't keyed. I broadly understand the +24V may be in part to speed up the pull-in of the relay; not clear to me whether the +24V is used to sustain relay hold-in.
Is there a schematic or mark-up out there for the 24VDC relay version?