B-Line Eaton series imperial cable tray and ladder, 4" Height, 14.97" Length, 18" Width, 18" and 6" Tray widths, Series 2-5 left reducer, Aluminum

It transitions an 18 inch wide tray down to a 6 inch wide tray, with the offset taken on the left side. The right siderail runs straight through and the left rail angles in, so the reduced section stays aligned to the right-hand rail of the original run. If you need the offset on the other side, you want the matching right reducer instead.

Handing on a tray reducer is called out looking in the direction of the cable run. The 4A-18-LR06 is the left version, meaning the taper is taken out of the left siderail and the right rail continues straight. Contractors commonly pick the handed reducer (rather than a straight/concentric one) when the tray has to hug a wall or stay flush with a cable drop on one side. Get the hand wrong and the reduced tray lands on the opposite edge of your support steel.

Aluminum. That keeps the fitting light at 3.6 pounds, which is why a single electrician can hold it up and bolt it off, and it keeps the reducer galvanically consistent with an aluminum tray run. Mixing an aluminum fitting into a steel system is a corrosion and hardware-match problem, so match material through the whole run.

Order Eaton B-Line part 4A-18-LR06. The number reads out as 4 inch depth, aluminum, 18 inch nominal width, left reducer to 6 inches, which is how you confirm you have the right fitting on a submittal or a takeoff.