The 031-326-RFX: This part carries a Not Recommended for New Design (NRND) status. That means the manufacturer is not actively promoting it for new projects, though it remains available for existing designs through the supply channel. For a new design, look at the standard-production BNC plugs in the same 031-32 base series; the RFX suffix signals a specific variant that may not have a direct one-to-one drop-in, so check the cable group and interface dimensions against your assembly before committing.
The 031-326-RFX is listed for RG-55, 142A, 223, and 400 cables — all 50 Ohm types with a similar dielectric diameter and braid coverage. It does not list RG-58, which is a common 50 Ohm cable but uses a smaller dielectric (0.116 in vs 0.150 in for RG-223). If you need an RG-58-compatible BNC plug, you want a different part in the 031-32 base family. The dielectric spacer is polyoxymethylene (POM) acetal, which holds its shape under the crimp force and gives consistent 50 Ohm impedance through the joint.