There is no such rank as "First Class Sergeant". You have either Sergeant First Class, which is E7, or First Sergeant, which, along with Master Sergeant, is E8.
YES
First Sergeant (E8).
First sergeant
The lozenge is simply a device to differentiate a First Sergeant from a Master Sergeant, Sergeant Major, etc.
I am in JROTC (junior reserved officers in training corps) so I have to know... Private Private first class specialist or a corporal (I am a corporal) sergeant staff sergeant sergeant first class Master sergeant or first sergeant sergeant major command sergeant sergeant of the army
US Army: 2nd Lieutenant is 2LT, 1st Lieutenant is 1LT, Captain is CPT, Major is MAJ, Lieutenant Colonel is LTC, Brigader General is BGN, General is GEN. Private is PVT, Private First Class is PFC, Corporal is CPL, Sergeant is SGT, Staff Sergeant is SSG, Sergeant First Class is SFC, Master Sergeant is MSG, First Sergeant is 1SG.
The United States army consists of twelve ranks. They are, in order of rank, Private, Private First Class, Specialist, Corporal, Sergeant. Staff Sergeant, Sergeant First Class, Master Sergeant, First Sergeant, Sergeant Major, Command Sergeant Major, and Sergeant Major of the Army.
When a First Sergeant leaves, it'll either be because they've been relieved of duty, or are about to be promoted to Sergeant Major. Who replaces the First Sergeant depends on the availability. It may be a Master Sergeant transferred from a staff position, or it may be a Sergeant First Class who is about to be promoted to E-8. You won't find Master Sergeants in line companies - Master Sergeants are found in staff positions.
The First Sergeant, or Top, is the senior NCO of the Company, Battery, or Troop.
"Buck Sergeant" is used in reference to a Sergeant (E5) - the lowest grade of Sergeant - as opposed to Staff Sergeant, Sergeant First Class, Master Sergeant, First Sergeant, etc.
Sergeant First Class is pay grade E7. In the Army, the next pay grade - E8 - has two ranks within it - Master Sergeant, and First Sergeant. The difference being that a First Sergeant is a company's senior NCO, whereas Master Sergeant tends to denote NCOs holding staff positions.