I just installed 0.50.0 and picked New Combat Vehicle, I didn't change starting year. Tried with Stardard and Advanced Tech Levels for Inner Sphere. Went to Equipment Tab at Top. Clicked on Filter Tab for Other, scrolled all the way to the bottom and it is last choice "Vehicle Sponson Turret" right after "Vehicle Mine Dispenser". I showed up for Tracked, Hover, and VTOL motive types. Didn't try any others. And I know in the past when I selected Vehicle Sponson Turret for equipment and added it the option and space to load it appeared under the Assign Criticals Tab. I know this also worked for me in 49.1 and 49.2 versions.