The K-1A model is compatible with 3/8"-24 threaded antennas and accessories.has an internal locking mechanism. This design requires more handling and machining than an external locking design, but offers weather protected components and additional security from casual thieves who may not be familiar with the push-n-twist locking mechanism. The K-1A can be used with any of our antennas up to 4ft  long and with all accessories too. The actual installed size is 2.9" (74mm) high x 0.600" (15mm) diameter. The upper and lower body parts are made of chrome plated machine brass. The locking pin and the spring are made from stainless steel.