Yes, the postman really did once ring twice. Sometimes he blew a whistle or knocked twice. The double ring and knock was the carrier’s signal to a household that the mail was being delivered. Before 1916 the Post Office Department did not begin to require houses to have mailboxes or slots. So when carriers delivered the mail to a home, they had to hand it over in person.
For more than a few carriers, knocking on hundreds of doors day after day wore on their knuckles. So they purchased wooden, dumb-bell shaped contraptions to use as door knockers on their daily rounds.