A thin constriction at a pressure antinode leaves resonant frequency unchanged

Evidence

At an antinode somewhere in the pipe away from the end, air approaches and recedes from both sides, causing pressure to rise and fall with no movement of the air. At such a place, a thin wall can be inserted with no effect, since the air is already stationary. — Eric J. Heller, Why You Hear What You Hear- An…, 1640