SRI Connector Gage Company. Model 23-923-3100-30.  3.5mm Female Jack to 3.5mm Female Jack. Flange mount.  DC to 34.5 GHz. 50 Ohms. VSWR 1.25 up to 34.5 GHz. Non magnetic 303 per ASTM A484 & A582 stainless steel. End to end about .977" and flange is .500" sqr. Hole center to center about .340". Hole 2-56 UNC-2B. Google for more specs if needed.  From our bins. Some may have been mounted and not used before being pulled back off. Some have never been mounted. These will mate to 3.5mm, 2.92mm and SMA. New these are high priced as much as $300 or more.