Over the course of the series, it is repeatedly established that Danny is the oldest of the four Reagan siblings followed in order by Erin, the deceased Joe, and then Jamie. There are a few certain episodes, however, where Frank mentions in conversation that Joe was the oldest.
Most of the police cars used in Blue Bloods are replicas, bearing the obvious wrong font for the 'NYPD' lettering on the side, having blue lights and bearing the modified NYPD logo that's also worn on the replica uniforms. But in several episodes you can see the replica cars mixed with real-life NYPD police cars. Most of the time the precinct number on the real-life cars are not shown, since the NYPD doesn't allow real-life precinct numbers being used in TV shows.
The Kahr K9 9 mm pistol was an approved off-duty/backup weapon from 1998 to 2011. It was pulled from service because it could not be modified to a 12-pound NY-2 trigger pull. From 2011, Danny Reagan couldn't be carrying a Kahr K9 9mm pistol.