I picked up a vintage Omega De Ville recently that was described as having a solid 9ct unishell case. Upon removing the movement, I noticed somewhat unusual hallmarks. It appears to be a case made by Omega Watch Co “OWC” for the English market; however, I didn’t find the “.375” that one would expect to see with a 9ct gold case. Instead there was a crown and the number 18. I did a bit of research and found some useful info at http://www.vintagewatchstraps.com/swisshallmarks.php which suggests that some 18k gold English cases did not contain a number indicating a percentage or millesimal fineness and that the mark for 18 carat gold was a crown and the number 18.

I’ll have the case tested to be sure but am I correct in thinking that this is likely an 18k gold case?

 

Is that white or yellow gold?

 

Yellow - I think I found the answer. It seems the purity/fineness of 18k cases made in London pre-1975 was indicated by an “18” and crown markings. From: https://webarchive.nationalarchives...ingcouncil.gov.uk/publications/hallmarks2.pdf

 

Also the “r” mark indicates that the case was produced in 1972, which is consistent with the movement serial number. It’s quite fascinating how much information can be obtained from the hallmarks of some cases.

 

Until you run across non standard ones. Lol.

 

Case made in the UK by David Shackman and Sons. London assay mark for 18k. As noted year letter indicates 1972. Can’t sleep!