Classic/Antique Car Repair: 84 chevt p/u master cylinder, chevt, diget

Question
got an 84 chevrolet p/u.went to part store to buy a master cylinder .they told me they needed a code off my vehicle .JB3,jb5,or JB1..for the correct one .cant find any numbers on the truck to match this code. looked on their computer they all looked the same. where do i go from here ?

Answer
There should be a decal on the inside of the glove box door that is a Service parts ID sticker. The number that they are asking for should be listed there. The next best is the 17 didget VIN. My local NAPA has been able to get parts for me using that. The 4th diget in the VIN is the GVWR/Brake system code and they should have that broken down in their manual. The 6th didget is the truck series, K for 4 wheel drive and C for conventional pick up. I hope that this helps.
Brad Sears