That is an iteresting item, and my curiostiy is jarred! I would think that if electricity is available, it's more efficient to just have an electric bell in most cases. So Jim_Ferer makes a good point about the steam bell being an alarm for over pressure. This is because it would act as an "audible gauge". So if the overpressure continued and increased, the frequency of the bell increases, and vice versa. Makes good sense
And if this is the purpose, the electric valve would be an override to stop an unwanted alarm, and/or as a test mode.
Steam power in itsself fascinates me, and this would be a great topic for the non-siren thread!
Charles