When were the 4903 series units introduced?

I’m curious to know when the iconic 4903 series devices were introduced. I know there are many models that start with 4903, but I’m referring to the self contained units (such as the 4903-9219). Does anyone know?

Sometime in the early-to-mid 90s I believe (maybe even the very late 80s).

The oldest date codes I found on them go back to 93