what do the different markings mean on upper recievers such as the "F"? and what are the others and there meanings?

Not sure about the F but there are forge markings on various uppers that denote what foundry produced the raw forgings. It has nothing to do with who machined & finished the upper. There are also F's on FSB's to denote the proper height for a carbine.

ETA: List of forge marks:

A (splintered) = Anchor Harvey Aluminum

AF = Alcoa Forge

C AF = Colt Alco Forge

C MB = Colt / Mueller Brass

Cardinal (stylized) = Cardinal Forge

CH = Colt Harvey Aluminum

CK = Colt / Kaiser Aluminum

Circle/Crooshairs w/"AR" = ArmaLite

CM = Colt / Martin Marietta

D (stylized) = Diemaco

DK = Diemaco / Kaiser Aluminum

E = Emco

EK = EMCO/Kaiser

E MB = EMCO/Mueller Brass

F keyhole = FNMI / Cerro Forge

FA = FNMI / Anchor Harvey

FK = FNMI / Kaiser Aluminum

FM = FN/Martin Marietta

FMB = FNMI / Mueller Brass

Keyhole = Cerro Forge

L = Lewis Machine & Tool

LK = LAR / Kaiser Aluminum

LM = LAR / Martin Marietta

M (under diamond) = Mueller Industries

PA = Capco / Anchor Harvey

PM = Capco / Martin Marietta