Brake linings confusion.

[Agv63]

Hi.

I'm from south africa, and have a 63 c10.
Rhd, and somewhere in its life more than 20yrs ago(i know this truck for last 20 yrs as it was my friends) it got gmc emblems.
Was a 283, then a 250,3spd and now a pontiac 455 with a switch pitch 400.
12 bolt rear, 17 spline (not for long, got some 5 lug replacements coming)
With press on backings, im searching for brake linings and found that up to 63 one part number and 64 on another part number, what is the changes between them?

Gonna try and upload some pics, as i see the trend here is pics galore😄.
Added g20 hd front arms and disc, and going corvette rallye wheels, still buzy with this conversion currently. Pics will be when it had drums and 6 luggers.
And some when i was finishing the 455 install.
Cheers,
Adriaan.