Jean Marcel’s Tantum collection has an invisible JM emblem on the watch face’s crystal that becomes visible only when breathed on.
It is a unique sign of authenticity for the German business that manufactures in Switzerland.
Tantum is a broad range of watches that have slim, flat, steel cases in common.
The JM A10 chronographs, which use the ETA 2894-2 automatic movement, come in at a height of only 11mm while the more dressy JM H05 watches, at just 6mm high, use the hand wound ETA Peseux movement.
A new 3-hand automatic this year, the JM A09, is 8mm thick and uses ETA‘s 2892A2 Élaboré calibre.
The watches look even slimmer because the circular cases are still comparatively wide at 42mm.
The collection is priced at around £764 (€1000).