The rank insignia for a First Class is an Eagle over 3 Chevrons, with the person's rating insignia between them. For collar devices, it's just the Eagle over the Chevrons.
In the US Navy, the ranks of "Seaman Recruit", "Seaman Apprentice" and "Seaman" are junior to the rank of Petty Officer 3rd Class. In increasing rank are Petty Officer 2nd Class, Petty Officer First Class and Chief Petty Officer. So, yes, any of the Petty Officer ranks are higher than any of the Seaman (or Airman or Fireman) ranks.
First Class Petty Officer, and a Machinist's Mate (a job rating). The MM stands for Machinist's Mate, and the "1" denotes that the person is a First-Class Petty Officer.
Petty Officer is abbreviated PO and the rank is usually included, 1st Class, 2nd Class, or 3rd Class Petty officer. E-4 is a 3rd Class Petty Officer, or PO3 E-5 is a 2nd Class Petty Officer, or a PO2 E-6 is a 1st Class Petty Offcier, or a PO1 CPO is a Cheif Petty Officer or an E-7 SCPO is a Senior Cheif Petty Officer, or an E-8 MCPO is Master Cheif Petty Officer, or an E-9 MCPON is Master Cheif Petty Officer of the Navy also E-9
Yes a HM3 is a third class petty officer. HM denotes that the person's rate (or job person is assigned) in this case it is a hospital corpsman. The Number following shows his pay grade. HM2 is a second class petty officer and HM1 first class petty officer. A third class petty officer is an E-4 which is the equivalent to a Marine Corporal which is also an E-4.
Cryptologist Technician First Class. A first class petty officer is paygrade E6,
petty officer first class
petty officer first class
petty officer first class
First Class Petty Officer is the Rank; the paygrade is E-6.
There were many more military positions in Colonial times. The go from, Crewman Apprentice, Crewman, Specialist, Petty Officer Second Class, Petty Officer First Class, Chief Petty Officer, Master Chief Petty Officer, Warrant Officer, Ensign, Junior Lieutenant, Lieutenant, Captain, Major, Colonel, Commander, Rear Admiral, and Admiral.
In the United States Navy, and E6 is a Petty Officer, First Class, or a "PO1"
USN PO1 or Petty Officer First Class, E-6, is equivalent to an Army SSG, or Staff Sergeant, E-6. The only difference is that Army SSG's are considered SNCO's or Staff Non-Comissioned Officers. Petty Officers 3rd, 2nd, and 1st class are not.