That seems to be when it became a common term for military dogs and doesn’t seem to have been for police dogs prior to that. The clever homophone seems to go back a least a bit further though, there was a brand of dog food ‘K-9’ in the '30’s for example (Google ngram picks up the term back to beginning of 20th century but looking at books seems many at least are paragraph K section 9 etc).
Note: Deebo seems to be recovering nicely.
*"“Deebo appears to be in good spirits and just wants to play with his little brother K9 ‘Emmett,’ however, due to restrictions Deebo cannot play or return to work under the doctor’s orders,” Saginaw police said…Police Chief Bob Ruth said he is a very popular dog with the community and at the department.
“Everybody loves Deebo.”* 
Just noting that in the military that is an outdated and unused term. I’m not sure when it was officially changed but the current term is MWD for Military Working Dog. The current MOS in the Army is 31K and is part of the Military Police Corps.