The $2500 bonus check Don gets is roughly equivalent to $22,000 in 2020 dollars.
In the flashback to where Don Draper grew up, the filming location is the same as Dwight Schrute's beet farm in The Office.
The hobo's symbol for a "dishonest man" is a stylized knife. In the Hobo Code it can be as simple as a sideways crescent, usually with a small cross (or just a dash) marked at one tip to symbolize the knife handle.
John Slattery was unavailable for this episode as he was busy doing his guest starring turn in Desperate Housewives (2004).
One of the guys at Midge's party chastises Don's work and mentions "ten dead kids in Biloxi". He is referring to an incident in 1959 in Biloxi, Mississippi where race riots over segregated beaches took place and ten people were killed.