VIN S versus VIN F

[CoachWagner]

Can anyone tell me the difference between the VIN S 6.5 and the VIN F 6.5?

Just for curiosity sake.

Thanks.

[gmctd]

Three versions 6.5L offered -
Normally-aspirated, no turbo
Turbo, egr for half- and 3/4 ton light duty, vin S, L56
Turbo, no egr for 3/4 heavy duty and one-tons, vin F, L65

All had oxidizing convertor, for burning off soot accumulations.