Napa doesn't seem to have a listing for the Cardone 10-1331 unit (dual master cylinder...not stock setup) I bought back in November 2008 either. I found it on RockAuto though.
http://cardone.com/English/club/memb...PARTNUM=101331
If you type in the GMC truck applications listed by the RockAuto website Napa lists the following new part number.
Brake Master Cylinder - Remfd
Product Line: Proformer Master Cylinders
Comments: w/ Bolt on Cover Master Cylinder Reservoir
NMC P36288
Price $31.99
Brake Master Cylinder - Remfd
Product Line: Proformer Master Cylinders
NMC P36246
Price $29.99
Brake Master Cylinder - New
Product Line: NAPA Ultra Premium Master Cylinders
NMC M36246
Price $75.49
Brake Master Cylinder - New
Product Line: NAPA Ultra Premium Master Cylinders
Comments: w/ Bolt on Cover Master Cylinder Reservoir
NMC M36288
Price $75.99
1967 M715 w/wn
1968 M715 w WVT993 maint bed
1969 M725