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Thank you! This matches what I was told in an Omegashop. They told me that the watch was made in Switzerland, but to avoid the customs costs, the imprint was omitted for trade in the UK and Australia.
UK produced cases generally have a full set of hallmarks - even for 9ct gold - but I believe other commonwealth countries, like Australia, often used the simple 9ct marking for these nationally-produced non-Omega gold cases.