C14 or C17 Pedal Rod Booster

[tucsonjwt]

I can't get an answer from the parts stores on the proper booster for my 1983 C20.

The options are:

Pedal Rod #C14 - GM Code JB6 Brake (7200 LB.)
Option Codes are Located on the "GM Service Parts Identification" Sticker

Pedal Rod #C17 - GM Code JB7 Brake (8400 LB.)
Option Codes are Located on the "GM Service Parts Identification" Sticker

I don't have a GM Service Parts Identification Sticker, so which one is correct for my truck? How would I tell the difference?

I don't see any markings on the booster or pedal rod itself. I do have a cast iron master cylinder, if that makes any difference.

Thanks.