Also in the original II and II Plus. On those older models, I think you need to type "F666G" at the monitor prompt to get to the mini-assembler. The "!" command was implemented in the enhanced IIe ROM.
I don't have access to my Apples at the moment, so can't check the details, but I'm now curious to know exactly what might have changed between various II+ - what did others gain that we were missing, or why did we have room for it when they didn't...