The MK VI horns were built entirely without blueprints and underwent considerable evolution over the production run. The employees simply remembered what to do and made changes as they saw fit. There was not a huge amount of difference between the French assembled and Elkhart assembled horns. Most of the differences were cosmetic. Here are the differences you will see during the production run:

Double S logo on neck ends around 55,000
Short alto bow ends 72,000
Medium bow alto ends 87,000
Long bow alto ends 135,000, medium bow returns
Flat plate neck mechanism ends around 155,000
Metal thumb hook ends around 133,000
Ball joint side keys begin 117,000
Body tube taper reduced at top end around 80,000
Serial number on neck ends 130,000
Strap hook location changed 200,000
Metal resonators discontinued 82,000
Neck key guide moved 155,000
Neck volume and ferrule size decreased 180,000
Knurling of bell to body ring discontinued 139,000
Nickel high G and E discontinued 148,000
Mk VI engraved above LH thumb rest 55,000 – 91,800
Mk VI engraved on bell to bow band 91,800 – 140,400
Stamped Mk VI on back body tube 140,400 – 143,500
Stamped Mk VI on bow band after 143,500

There may be other differences which crop up from time to time……the lowest number Mk VII I have seen is 231,000 ( I owned it for several years), the highest number MK VI I have seen is 261,000……