Speaking as a historian, not an officer, it appears that the policies of different agencies varied significantly.

For example, once NYPD began buying M&P’s and Colt OP’s directly from the factories in bulk for resale to their officers, no 5” or 6” barreled guns were available. They all came with 4”. That would have been in place about the end of WW II at the latest.

On the other hand, the California Highway Patrol specifically required a 6” barreled revolver from the time that regulations first appeared in the 1920s until 1976, when, with the introduction of the Models 67 and 68, the 4” became available, and other approved guns from Colt, S&W, and Ruger were also allowed with a 4” barrel.

Since, by definition, most CHP officers spent their patrol time in cars for several decades before 1976, I’m not sure any change from foot beat to vehicle patrol was a factor here