The part is usually called a retaining ring (or lens board retaining ring). For large-format lenses, it generally matches the shutter size and lens board hole, not the lens name itself. In other words, you need a retaining ring that fits the shutter your Sironar-N is mounted in and the board opening you’re using. Common shutter sizes are things like Copal #0, #1, or #3. Once you confirm the shutter size, search for that specific ring—for example, “lens board retaining ring Copal #0” or similar. These are commonly found from large-format parts sellers and on marketplaces like eBay.