I should correct myself this part is not a casting it looks to be a very thick piece of aluminium (ca. 4mm) and has steel bolts at the bottom edge. The top has been cut off.. the outer edges have been turned over to form a flange for bolting to a lower surface. The inner edge is flanged but curved over.

Even though the numbers suggest a 504/Harrow –

I cannot see this piece being used in a a 504K.

The Harrow had an interesting but different structure:

http://www.iwm.org.uk/collections/item/object/205210465

http://www.vord.net/609/harrow.htm