Short answer
"Fake Audemars Piguet Frohnhofer" is not a term used by Audemars Piguet, by auction houses, or by authentication services. No Audemars Piguet model, calibre, or case code carries the name Frohnhofer. Search results that combine the two words return forum posts, marketplace listings, or mistyped surnames, not factory records.
Audemars Piguet was founded in 1875 in Le Brassus, Switzerland. The brand catalogue lists Royal Oak, Royal Oak Offshore, Royal Oak Concept, Millenary, Jules Audemars, and Code 11.59. Frohnhofer appears in none of them.
Where the word may come from
- A seller name or storefront handle
- A misspelling of a German or Austrian surname
- A misheard dealer or brand name in a video
None of these make a watch genuine. The watch itself carries the proof.
How to check a Royal Oak
- Read the caseback. A genuine Royal Oak shows an engraved reference number and a case number. Both must match the papers or an extract from the Audemars Piguet archives.
- Measure the case. A Royal Oak Jumbo 5402 is 39 mm wide and 8.1 mm thick. Many fakes run 1 mm to 2 mm thicker.
- Check the calibre. Reference 5402 uses calibre 2120, derived from the Jaeger-LeCoultre 920. Later references use 2121, 3120, 4302, 7121, and others. Genuine rotors and bridges show Geneva stripes and beveled edges.
- Look at the dial. The Tapisserie grid is stamped and cut. Replica grids show shallow squares and soft corners.
- Check the bezel screw heads, the crown, and the crystal edge. On genuine later models the bezel screws sit flush and the flat slots line up in one direction.
- Ask the brand. A service center can open the case and confirm the movement. Christie's and Sotheby's do the same before a sale.
Reference numbers to know
Royal Oak 5402 (1972), 15202 (Jumbo extra-thin), 15500 (41 mm), 16202 (50th anniversary, 2022). The 50th anniversary models use calibre 7121. A Frohnhofer label on any of these means nothing without matching numbers and a movement check.
Limits
No public source ties the word Frohnhofer to Audemars Piguet. If a listing uses it as a model name, treat the listing as unverified. The reliable checks are the caseback numbers, the movement, and a written statement from the brand or a qualified watchmaker.