Rolex with no serial engraving?

[jlmnjem]

I'm trying to determine the authenticity of a watch that belonged to a relative who passed several years ago. He bought it from his brother probably 30 years ago, which is the only history I know. I've removed the bracelet and there is nothing engraved between the lugs on either side of the case. I know nearly zero about watches, but I thought all Rolexes would have the serial number engraved on the case. Opinions, please? Thanks.

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