2120 is the NGK stock number for the part number D8EA NGK spark plug which is correct for a CB350F.
http://www.ngksparkplugs.com/apps/motorcycles/make4.asp?id=1101&type=regDR8ESL is a resistor plug, and will work in your bike, but are not, technically, the correct plugs if you are still using the stock resistor plug caps.
You may experience shortened plug life and effectiveness using these plugs with the stock ignition coils, but it won't hurt anything. The extra resistors lower the spark voltage margin provided by the stock coils and ignition. This comes into play when the plugs approach end of life where they require more voltage to fire with vigor. You will likely have to replace them at shorter intervals than the correct spark plug without resistors.
However, if you change to non-resistor spark plug caps, the plug life is equivalent.
Your choice.
Cheers,