When something is described in the manner [brand-name] [number] [object] (EG: Boeing 747 landing-gear) then when the brand-name is in all capitals (EG: MOS 6502 assembler) LT erroneously complains that the plural should be used.
The cause of the problem is that the upper case brand name is not a proper noun (it does not have the postag NNP). The correct brand name is 'MOS Technology', so you will still get an error with 'MOS 6502 assembler'.
'MOS Technology 6502' is the specific processor.
'MOS 6502' is the instruction set.
EG: the MOS Technology 6507 (of Atari 2600 fame) is a MOS 6502 processor,
but that does not make it a MOS Technology 6502.
(compare Intel 80386 vs. IA-32)
@SkyCharger001, thanks for the clarification.
Now, LT gives no warnings for 'processor' in these sentences:
The MOS Technology 6502 processor is an 8-bit microprocessor.
Figure 1 shows a MOS 6502 processor in a DIP-40 plastic package.