Harrier Designations

I am confused with the designations of the Harrier family as defined by the Tangmere Aviaton Muisem: could somebody clarify the designations of the Harrier family

The original Harrier put into squadron service were Harrier GR1, (later upgraded to Harrier GR3)and AV8A by the USMC.

Tangmere Aviation Musem identfies them as Harrier FRS1
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I believe almost all of the the Sea Harrier fleet was upgraded from Sea Harrier FRS1 to Sea Harrier FA2

At Tangmere the aircraft is identied as a Sea Harrier FRS2
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The Harrier II was classsified by the USMC as the AV8B: in the UK it becamre the Harrir GR5 and depending on the updates it became either a GR7 or a GR9

Please could somebody clarify these designations
Dai Lee