In A Corner of the Universe, what did Nana keep hidden under the rug in the dining room?

In “A Corner of the Universe” by Ann M. Martin, one of the characters is Harriet Mercer. She is Hattie’s grandmother, and Hattie calls her “Nana.” Nana is well-to-do and has a large house with servants. She is strict and rigid with clear views of the right and wrong way to do things. She has high expectations for proper behavior.
The author does not explicitly state that Nana has something hidden under the rug in her dining room. However, there is a scene where Hattie is describing a time when Nana has come to their house for dinner. She is expressing that she is aware why Nana does not come over for dinner often. She states that she believes Nana disapproves of their attire and the fact that they do not have a maid. She says, “No little buzzer is hidden under the rug by Nana’s foot, a buzzer with which she can silently summon the maid from the kitchen” (Ch 3). From this, we can assume that at her own house, Nana has such a buzzer installed.