Accessory mounting kit for ALP600 or SNA600 antennas. For details, see page 2 of the PDF under More Product Info link at bottom of page.
The ALP600 and SNA600 both include a 50 Ohm BNC connector and a 3/8"-16 stainless steel mounting stud. The mounting stud adapts these antennas for use with the optional mounting adaptor kit.