There certainly was a run of 3" Model 19s, done for a police spec. contract (Michigan State Police?), which are considered to be very rare, and some 5" Model 19s, which are considered by collectors to be rare. There was also a much more recent run of 3" Model 19s, possibly for Lew Horton, but those don't command the prices that the original 3" guns do.
Supica and Nahaus make no mention of a 3.5" Model 19, nor does Jinks, so it would have to be something special, possibly a one-off prototype gun, or perhaps even a "lunch box" gun, made up by an employee and taken home in a lunch box.