CREATE INDEX receiving_item_delivered_received ON receiving_item_delivered_received USING btree ( eventtype, replenishmenttype, serial_no, eventtime DESC );
More work_mem as Tomas suggests, but also, the above index should find the candidate rows by the first two keys, and then be able to skip the sort by reading just that portion of the index that matches
eventtype='LineItemdetailsReceived'
and replenishmenttype = 'DC2SWARRANTY'