Description: Gibson J-45 flat top guitar Available: August 1942 to present. Collectibility Rating: WW2 war-time 1940's models: B+. Late 1940s/early 1950's models: B. Late 1950s model: B-. 1960's models: C-. General Comments: Initially, the Gibson J45 guitar differed only slightly from the discontinued J-35. Internal contruction changes from the J-35 included top bracing strenghtened slightly with