Radar mast type... ie: Supported versus unsupported. Some conning towers have slots for the mast to ship through, while others do not, so some radar masts are "smooth" (unsupported), and some have hardware "framing" supports to hold it to the conning tower.
Look at your radar mast install, and see if your conn doesn't have supports for the "smooth" style radar masts, as seen 3rd & 4th from right above, and you might not have the 1st or 2nd radar mast on the right instead... maybe the 5th or 6th... ?? The conns are a mess in the Stock game, and were not fixed in RFB, or most other mods...