The E-23 rubber o-ring acts as an inner barrel stabilizer on all LM4 PTR GBB rifles, as well as the entire AKG Series. It appears on respective diagrams as LM4 PTR part 44-0, and AKG part 63.
The F-18 spring fits multiple KWA models, and appears on respective diagrams as M9 1st Gen part 52, M9 PTP and M9 Tactical PTP part 74 and M93R II NS2 part 103.