As I've written many times, it's my considered opinion that the commanding position of the Selmer Mark 6 was largely because it was the most recently released major new design on the market when rock and roll began its march to dominance and largely killed saxophone in popular music.
The MkVI a success because of global politic. With the Marschall Aid it was possible to import high quality saxes to USA to a low cost. The MkVI reached the market just in time. Conn and Buescher trough in towel and concentrate the production to make student saxes. USA was a high cost country after WWII.
Mk VI was a reliable saxophone. The tone and sound fit the music industry in a good way. The sax tech liked the mk VI because it was rather easy to work on. The musicians liked the mkVI becase it had a neautral voice. They could get more well paid, compared to be on the road, session gigs. They could use a MK VI to classic, jazz, blues, R&B, Rock, Pop, Schlager, commercials ..... . A mainsteam sax perfect with a commercial tone!!! All this to a low price.
In the early R&B the MkVi was not that frequently used. Ok, King Curtis and Red Prysock played Selmer saxes. But Big Jay Mc Neely, Willis Jackson, Paul Williams, Lois Jordan, ... were Conn men. Chuck Higgins, Joe Houston, Al Sears, Johnny Griffin .... were playing King saxes.. When "Rock & Roll" cme in mid 50's, Lee Allen was on Buescher, Jim Horn and Steve Douglas on King tenors .... .