The Surmen K505M-NMO. A NMO style magnet mount.
This heavy duty magnet mount features Surman's "Super Magnet". It is a strong holding magnet for antennas, and has a removable PL-259 connector for easy installation.
The base is about 4.5 inches across, and has a rugged rubber pad with a lift tab. It has a 4 meter long cable.