As anticipated, these are marines and 192 was a recruit training platoon. The photo would have been taken at San Diego, California or Parris Island, South Carolina. Your Grandfather was apparently a drill instructor. The fifty-odd men pictured would have been parceled out among units of the Fleet Marine Force (FMF), the Fleet and base units throughout the marine corps. Send me your Grandfather's full name and date of birth and I'll see what I can dig up.