Sgt. Fred 'Frog' Catteau is seen wearing a Senior Sergeant rank insignia in the first episode but wears a regular Sergeant rank in every other episode.
Sr. Sgt. Peter 'Ridgy' Ridgeway is performing the role of a Senior Sergeant within the squad but wears a Sergeant rank insignia. The script mentions that Sgt. Fred 'Frog' Catteau is the senior member.