The dagger used is "keris" the traditional dagger from Indonesia.
Martin Balsam asks the counterman if rat poison is painless. This is similar to a background conversation in the hardware store scene from Hitchcock's recently-released Psycho (1960), which he also appeared in.
Elsie tells her husband that the doctor said she could live another thirty years. Vivian Nathan, the actress who played Elsie, lived another 54 years, dying in 2015 at the age of 98.