Here's some interesting info about female jurors in Massachusetts
Women did not become eligible for jury service in Massachusetts until 1950. In 1920, soon after the Nineteenth amendment to the United States Constitution enfranchised women, the Massachusetts House of Representatives asked the Supreme Judicial Court for an advisory opinion regarding whether that amendment required women to be eligible to serve as jurors. The Court responded that women would be eligible for jury service only if the legislature enacted a new law that specifically provided for women jurors.
Following lengthy legislative and political battles, Massachusetts enacted in 1949 a bill making women eligible for jury service. However, this law permitted a woman to choose to have her name omitted from the jury list and to be excused from rape and child abuse cases if she would likely be "embarrassed" by hearing the testimony or discussing it in the jury room. When this limited-eligibility law went into effect in 1950, Massachusetts was the 39th state to permit women to be seated on a jury.
In 1975, the United States Supreme Court ruled that excluding women from the jury pool violated a person's right to a fair trial by a representative segment of the community. In 1979, the Supreme Judicial Court ruled that particular traits, including race and gender, could not be used to strike potential jurors. This landmark decision, Commonwealth v. Soares, is considered to mark the end of permissible gender bias in the selection of jurors in Massachusetts.
So, it was not until 1975 in Taylor v. Louisiana, when the U.S. Supreme Court overturned the 1880 Strauder decision by ruling that states could not exclude women from jury duty based on sex alone, that women were allowed to serve on juries in all states.
So, your assumption about what went on in the mid-50's is probably correct. Some states might have allowed women to serve on juries, but not other states. But, by making the jurors in Twelve Angry Men all male, the playwright, Reginald Rose, had a jury composition which would seem familiar to all viewers in the audience, including those who lived in states which did not permit women to serve on the jury.
The play Twelve Angry Men is set in a jury room in a court in New York City, and Rose based it on his own experience as a juror in NYC on a manslaughter trial. Women were allowed to serve as jurors in New York at the time the play is set, but, as you already noted, they were often granted exemptions not given to men, so the number of women actually serving on juries might have been low.
The playwright choose to have an all male jury for his dramatic purposes. He could have accurately included women on his fictional jury of that time, but he chose not to.
Thanks for raising the question. It made me think about something I never really questioned before, like why that jury in Twelve Angry Men was all male. I simply assumed it was only for dramatic effect, to eliminate gender issues from the conflicts among the jurors. But, I do think you are right, it also reflected a common jury composition during the early 1950's.