You are absolutely right. That distinctive circular scar—typically slightly raised, puckered, and smaller than a dime—is indeed the hallmark of the old smallpox vaccine, specifically the ones administered from the late 1800s up until the early 1970s in the U.S. (routine vaccination ended here in 1972) and a bit later in other countries.
To add a little more depth to your excellent summary:
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The “Bifurcated Needle”: You mentioned the “double-pronged needle.” Its official name is the bifurcated needle. It was designed to hold a single drop of the vaccine between its two prongs via capillary action.
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The “Scarification” Method: The goal wasn’t to inject the fluid, but to deliver the live Vaccinia virus (a cousin of cowpox) directly into the dermis—the living layer of skin where the immune system has a dense network of cells. The repeated pricking created a small pool of the virus right at the surface of the dermis, which provoked a much stronger and faster immune response than a deep muscle injection would have.
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The “Take”: Doctors and nurses didn’t consider the vaccine successful unless it produced that blister and scab—they called this a successful “take.” If it didn’t leave a scar, they assumed the vaccine didn’t work and would often revaccinate you.
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Why the scar is circular and dimpled: The scar isn’t just from the wound itself, but from the body’s immune system attacking the virus-infected skin cells. The resulting inflammation and localized tissue death caused a mild form of atrophy (thinning of the skin) in that exact spot. The puckered, dimpled look comes from the scar tissue contracting and pulling inward as it healed, tethering the skin to the underlying tissue.
A fascinating footnote: Because of that very recognizable scar, for decades it served as a de facto biometric identifier. In the 1960s and 70s, if you lost your vaccination card for international travel, customs officials in many countries would simply look at your upper left arm. If they saw that circle, they knew you were safe to enter.
Today, the only people in the U.S. routinely receiving this old-style, scar-leaving vaccine are military personnel and certain lab workers handling orthopoxviruses. The modern smallpox/monkeypox vaccine (JYNNEOS) is given as a standard subcutaneous injection under the skin and, as a result, does not leave that famous circular mark. Your scar is effectively a fading piece of a global public health victory—the eradication of a disease that killed 300 million people in the 20th century alone.