Why are there different lacquers on some older Mark VI models ?

Up to the  Mark VI, instruments were shipped unfinished to Selmer-USA (now Conn-Selmer, a division of Steinway Musical Instruments).
Padding, lacquering, engraving and finishing was done in the Elkhart plant. The American lacquer was a darker, red shade than the clear lacquer used in the French factory. Only horns for the US market followed this process, balancing out a supply chain shortage in the French factory during the post-war period.
From the Mark VII onwards (1973), all saxophones have been delivered 100 % finished by Selmer-Paris at the Mantes factory.
Today, Selmer Paris instruments have the same lacquer, engraving and high standard of finish throughout the world.

Why did Selmer Paris stop the Mark VI model?

It was time for the instrument to evolve. Even if this model is thought of today as a great collector’s item, it had its imperfections and was in need of changes. After almost twenty years of existence, it was time for Selmer Paris to bring some improvements to its manufacturing in terms of tuning, emission, sound richness, homogeneity and so on. Also, in the early seventies, many Japanese manufacturers entered the saxophone business.
This, in conjunction with a more price-conscious market, created a need for greater efficiency and more industrialized processes in the factory. Though the rupture with the Mark VI was not initially well accepted by the professional market, it appears the Mark VII has been a decisive move in the long run for Selmer Paris. Without question, the sound quality of the present models has benefited from changes wrought over the last thirty years, and Selmer Paris continues to fulfill its mission as a manufacturer dedicated to improving the instrument.