Question about hallmarks on a vintage rose gold Omega watch

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Hi everyone, just bought this beautiful rose gold omega from a watch dealer. The photos are missing the second hand cause it was misplaced, but has been found already. My question is regarding the case has no indication of 18k gold but it has gold hallmarks all over the exterior, the lugs, the omega logo. Any advice on the matter is deeply appreciated.

Here are the specs from the dealer:

Brand: Omega;
Model: Automatic
Movement: Omega automatic cal. 501 - 20 jewels movement (http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&&2uswk&Omega_501);
Case diamater: 33.5mm (excl. crown);
Case material: 18k rose gold (one can see the eagle head on the caseback, the French hallmark for 18k gold);
Dial: original (untouched) two-tone dial with Arabic numerals at 12, 3, 6 and 9 ;
Hands & crown: original Omega dauphine hands and Omega crown;
Crystal: plexiglass;
Strap: brand new black leather strap with stainless steel pin buckle (rose color, same color as case);
Lug Width: 18mm; and
Year: 1956 based on movement serial number 15,XXX,XXX.
 
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French National case with hallmarks and no "18K" mark quite typical.