There is a 1963 Charlie Persip ad (could have been shot earlier)on the Gretsch website showing him playing a "natural polish maple" kit in 14x18; 8x12; 14x14; 14x5.5 sizes. That's the earliest verified date I know of for a natural maple finish.

http://www.gretschdrums.com/ebooks/artist_ads/index.html#/30/