Vincent Bach started making trumpets in 1925. From that time through the late '70s, overall construction for every Bach Stradivarius trumpet had one thing in common: two-piece valve casing construction.
Bach made their valve casing on all trumpets out of two-pieces, the bottom two-thirds out of brass and the top third, baluster, out of nickel-silver (and on some occasions bronze). Typically, the lower constructed casing unit would be run through a 'mill' to create the valve guide slots. Then the balusters would be soldered on top to (cover) complete the casing assembly. Two-piece valve casing construction was the typical technique at the time. However, in the late '70s, that changed.
Because of the difficulty to machine valve guide slots in the casings, The Selmer Company invested in one of its' first Computer Numeric Controlled (CNC) machines. These machines had live tooling which could reach into a one-piece casing and 'whittle' out the valve guide slot. Because of the introduction to new technology, all the 180 series Bach trumpets started going to one-piece valve casing construction from the late '70s up until 2010.
2010 marked the introduction of the 190 series with the Artisan B-flat trumpet AB190. This switch marks a shift back to the original construction techniques of using two-piece valve casings. During product development exercises with Bach Trumpet Artist and Principal Trumpet of the Cleveland Orchestra, Michael Sachs and the team preferred the two-piece valve casing construction.
They found the 190 series two-piece valve casing typically provides the player:
Since then, all new trumpets on the 190 Stradivarius series including the Artisans have been using two-piece valve casing construction. Ultimately the difference between the 180 and the 190 series is simply the construction technique used in the valve casing construction.