I've used 3M 6001 and 60923 and can't tell a difference between the two other than the 6001 breathes a bit easier. This doesn't mean that either is the correct cartridge to use though. I also don't use a half mask, I use the 6800 full face along with a paint suit. Not even close to as good as a fresh air respirator but it is what it is. I guess the isocyanates can absorb into your skin, so want to keep that protected as best you can.
Go to any automotive paint store, and they may have three different companies with dual barrage respirators; buy the one that fits you best.
DO NOT buy one at a big box store or online as it may be the same brand but may be made for welding or varspare type materials.