The NATO strap (originally British MoD G10 NSN 6645-99-124-2986) is the canonical military pass-through strap: a single piece of nylon that threads through both spring bars and a separator strap, with flat metal buckle and keepers. The NATO became a mainstream watch-enthusiast strap option from the 2010s onward; James Bond on a NATO in the early Sean Connery films is the cultural origin point.
The Zulu strap emerged in the 2000s-2010s as a market-positioned alternative for tactical and military buyers seeking heavier, more robust construction. The name is a play on the NATO phonetic alphabet: NATO straps are sometimes informally called "Alpha" straps; "Zulu" is the next letter convention used to distinguish the heavier variant. The naming has no formal mil-issue source; Zulu is a commercial product positioning, not a NATO procurement specification.
"NATO is for the smaller wrist and the smaller watch. Zulu is for the larger wrist and the larger watch."- Strap retailer on the NATO/Zulu choice
The physical differences from a NATO: (1) thicker, denser nylon fabric (~30-40% heavier); (2) rounded buckle and keeper hardware (NATO uses flat-stamped metal); (3) typically polished or PVD-coated buckle (vs NATO's standard brushed steel); (4) often single-piece without the separator strap of military-spec NATOs. The result: a heavier, more substantial strap that visually balances larger watch cases (44mm+ tactical pilots, 47mm+ Panerai-style tool watches).
Use cases: Zulu straps pair with tactical / military / dive watches. Sinn EZM, Bell & Ross BR-X1, Panerai Submersible, military-aesthetic pilot watches frequently use Zulu straps as alternative configuration. The strap reads heavier and more aggressive than a NATO; it is out of place on dress watches or smaller dress-aesthetic pieces (where a NATO is also out of place).
For buyers, the practical guide: NATO for casual / vintage / smaller cases; Zulu for tactical / dive / larger cases where the heavier construction visually balances the watch. Aftermarket Zulu straps are widely available from specialist suppliers (Crown & Buckle, Stitched, Erika's Originals heavier variants) at USD 30-80; the price differential vs NATO reflects the heavier materials. Most modern microbrand divers ship on FKM rubber (the modern dive-strap default) but Zulu remains a strong tactical-aesthetic alternative.