Sea Mattress an LCT(R) Landing Craft Tank (Rocket) carrying 1060 3 inch RP rockets that could be fired in 40 round broadsides or all in one ripple fired BROADSIDE. I think 1060 rockets justifies the bold capitals
Sea Mattress wasnt very accurate it was aimed by dropping 3 anchors and then using the windlasses to pay out or haul in the anchor cables to point the whole vessel, Radar was used during the aiming in poor visibility. Elevation I am not sure about but I imagine it was fixed or only adjustable during building.