Charlie's middle name is Edward.
This is the second time that Don has had his gun stolen, the first time is in the second episode of the first season, Uncertainty Principle (2005). As he says this one is his second and he's had it six years, this gun is likely the one that replaced that one.
Don has been an FBI agent for 15 years, and worked with David for six years.
Last show of the series.
In the episode it is said that Don has been an FBI agent for 15 years. With his birth date being 15th July 1967 that would make him 43 years old by the time of the show and 28 years old when he started. However in the episode Friendly Fire (2009) his FBI mentor talks about working with him when he was a green 25 year old, so it would be 18 years as an FBI agent.