Read 垫圈 as diànquān ("cushion ring") for a washer/gasket. The same characters read diànjuàn mean "to spread litter in a pen" — a farming sense you'll almost never see in modern speech.
memory
垫 (cushion) + 圈 (ring) = a little ring that cushions the bolt against the surface. Picture the rubber O‑ring under a faucet.
