Tthe Beretta M9 wasn't developed until 1976 and didn't enter service in the US military until the 90s, where as the beginning of the movie takes place in 1972. Also, the M16 that Peniston had just before letting the Creature's cage go was the A2 model, which wasn't developed until 1981 and didn't enter service until 1983.
After Dr. Chase brings the injured Constable back, he is giving a monologue about black magic to his daughter. The portion where they zoom in on her face, you can see that the Constable's lips are not moving even though the monologue is still going.
When Dr. Chase is bringing the injured Constable back from the marshes, the truck is left hand drive. Earlier in the movie when the Constable demands that his daughter get in the truck it is right hand drive.