The easy way is 39mm you have a 5402 Royal Oak Jumbo and 36mm you have a Mini Jumbo 4100SA

The 4100SA indicates Steel & Gold (“SA”=acier/acier‑or) version of that reference.
It is noted as having a non‑screw‑down crown, which means its water‑resistance is more limited.
Typical dial signature: at the 6 o’clock position it says “Swiss” only (at least in early examples).
Production years ~ late 1970s into early‑80s (for this mid‑size line) for the 4100 series.
Calibre 2123 (based on modified Jaeger‑LeCoultre movement) for the early 4100 models.

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AP Royal Oak Ref. 5402: Confirmed Series Production History & Dial Variations

By: Bill : November 12th, 2025-19:24

Introduction The Audemars Piguet Royal Oak 5402 “Jumbo” is one of the most iconic watches in modern horology, but its early production history has long been misunderstood. This article consolidates findings from WatchProSite contributors and AP archival s...

Thanks for sharing all of this information, and linking the other posts. I am a novice on the 5402.

There is always more to learn about the 5402. The problem is trying to get to the facts which is never easy. Thanks

By: Bill : November 13th, 2025-15:55

Mine doesn't have an A prefix

By: cazalea : November 13th, 2025-00:05

It is steel and gold two-tone. But I don't think it's a Jumbo size. How would I know if it's a 5402? do I have to read all those linked files to find out or will someone just shout at me "You dope, that's a XXXX not a 5402" Ignorant of the ways of AP Mike...