Extralite UltraStar 4 expander (original Specialized expander plug is 28g).
Extralite ExtraBolt 3.6 Cassette Lock ring (Ultegra lock ring was 5g).
I spotted these Extralite bits on eBay, then managed to find the local seller on FB Marketplace. He gave me a good deal for taking both so I pounced.
Weight has gone down from 5.60kg to 5.57kg.