The Q3848420 is the steel Reverso Classic Large Duoface: a Reverso with two dials. The front shows hours, minutes and small seconds; on the reverse, visible as soon as the case is swivelled in its carrier, a second dial shows a second time zone. You can wear whichever side you prefer facing up.
Both dials are driven by a single hand-wound movement, calibre 854A/2. The case is slightly larger than that of the Monoface, because there is a dial and a crystal on both sides.
On a pre-owned Duoface we check the swivel mechanism as on any Reverso: play, wear on the sliding rails and the pivot, and a latch that holds the case firmly in both positions.
We also assess both dials and both crystals, since the reverse is often treated with less care, and we test the setting of the second time zone. The gadroons quickly show whether the case has ever been polished inexpertly.
For a Reverso with a solid back there is the Q3858520; see also our guide to buying a pre-owned watch in Belgium.