Kowalski is by far the most popular surname in Poland, practically like "Smith" which as a matter of fact it means, in adjective form usual in Polish names. (Kowalczyk - Marilyn Monroe's name in Some Like It Hot (1959) is closely related to it - meaning "Smithson"). The name "Kowalski" identifies a person as a Pole.
Walt Kowalski's gun collection seems to consists of weapons he used in the military. His rifle is an American M1 Garand, a 9.5lb .30-06 gas-operated rifle. It was first issued during WWII, then re-issued in Korea before being phased out by the M14 selective fire .308 rifle. His pistol is an M1911A1, a .45 ACP semi-automatic handgun also issued during the Korean war.
The "cool medal" the kids find in Walt's basement is the Silver Star, the U.S. military's third highest award for valor in combat. Despite its predominately gold color, it gets its name from the smaller silver star (based on the small silver World War I Citation Star) set inside the large gold star.
When Walt is at the Hmong's party, he pats the head of a young Hmong girl passing through, causing the family members to audibly gasp. In Hmong culture, the human head is believed to house the soul, and any touching of the head is believed to jeopardize this, and is thus considered very disrespectful.