From messybeast.com, two old enclycopedia entries.
Note how the “retriever” entry shows a dog that we would now call a flat-coated retriever. From 1900 to the First World War, the flat-coat was in its halcyon days. At this time, the golden was part of the flat-coat breed, referred to as “Tweedmouth’s strain,” although Colonel Le Poer Trench’s golden dogs were registered and promoted as Russian retrievers.
Note how the 1923 entry of depiction of a retriever appears to have some wave to its coat. Fanciers had decided to breed out the wave in the wavy-coat. In flat coats, the wave has nearly been bred out, but the golden retriever, which was separated from the flat-coat before this waviness disappeared, still can come in a wavy coat.