The postcard's back promises, "Every room with bath, running ice water and Servidor. Rates: Single $2.50 up; Double $4.00 up; Twin Beds $5.00."
In case you were wondering -- and I was, so I looked it up -- a servidor was:
. . . a compartment built into a guest room door. Each side of the compartment had a door and a signal. A guest could place an item of clothing needing pressing into the compartment and a hotel employee would open the servidor from the hall to collect it. It could also be used to deliver items to the guest without disturbing them. The servidor's most praised service was its removing the need to tip employees."
Thanks to Forgotten Detroit for the definition.