Margaret Tobin is a common Irish name. Margaret Tobin Brown of Titanic fame was born in Hannibal, MO. She had lived in Colorado, Newport, Rhode Island and Europe before making her final home in New York City. Rockville is northeast of Hartford. I find no evidence of her having resided there, and certainly not before she was married. However, perhaps one of Margaret Brown's great-granddaughters will see your post and confer with you? They sometimes visit this site.
I will be happy to compare handwriting if you'd like to send me a scan of the signature.