The metal rail bar mounting plate with Vixen style dovetail is compatible with all standard Vixen style saddles manufactured by any brand. Two 3.5" (87.6mm) countersink slots are for balancing equipment installation. 0.25" (6.4mm) wide, the slots can hold M6 and 1/4" screws. Each slot also features three 3/8" screw holes (M8 countersink holes) with a 1.4" (35mm) spacing in between. Twenty 1/4" screw holes on both sides of the countersink slots have 0.8" (20mm) spacing between each other. These holes are useful for mounting a variety of devices with the matching thread. Scale markings help you find a precise position to mount equipment for astrophotography, celestial observation, night sky watching, and stargazing. Soft nonslip rubber pads increase stability while protecting the device from unwanted scratches. Four safety screws at the bottom can prevent the mounting plate from dropping. SpecificationsModel: QR003 Material: Aluminum alloy Mounting Threads: M6, 1/4", 3/8" Dimensions: 9" x 1.7" x 0.6" / 230 x 43.5 x 15mm Weight: 7.7oz / 219g Package Contents2 x 3/8" Slotted Screws 4 x 1/4" Hex Socket Screws 4 x M6 Hex Socket Screws 1 x 4mm L Shaped Hex Key 1 x 2.5mm L Shaped Hex Key