Feed water trays came in, I believe, when Churchward added top feed to the safety valve fitting on the boiler barrel. The trays were either side of the main steam pipe and supported on brackets which had a semi-circular shape to fit over that pipe. Feed water trays are about 1 1/2" deep, six or so inches wide and extend from the safety valve on the barrel almost to the front tubeplate - the top of the sides of the trays have scalloped edges.
The dimensions of the trays are such that the water feed evaporates before hitting either the boiler plates or the surface of the water below. Any impurities in the feed water are retained on the trays.
Whilst all of the feed trays and brackets are made of steel all of the nuts and bolts to fasten the parts together are (ought to be) brass.
Feed trays are inserted into the barrel one section at a time, the trays slid forward and the next section bolted on. Removal is the reverse sequence. Feed trays would have been removed on a regular basis so as to remove the scale from the trays.
regards, Graham