Jack Lord appears in the film 3 times, twice with lines, as one of the lab technicians at police headquarters.
Replacing the lead shot in the shotgun shell with sand would actually help the investigation. A shotgun barrel is not rifled with grooves, like a rifle or pistol, but is smooth so therefore the barrel would not leave identifying marks on the lead pellets. However, the type of pellet could identify the type of shell and possibly the type of shotgun used.
When Dr. Mahan looks up "Fisher Monuments" in the phone book and says it's on "Gun Hill Road" in the Bronx. However (and as noted here in "Filming Locations"), the monument company is actually on East Tremont Ave, also in the Bronx. A wide view taken from inside the yard towards the street when the police arrive shows St. Raymond's Cemetery in the background. This monument company was also used for a similar shootout between police and the bad guy in Naked City (1958) about ten years later.
The newspaper Frank tells Mary to look at is the Daily News (New York, New York), dated 16 Aug 1949.