Determining which DDR3 sticks go together in a kit

Hi everyone, I need some help about identifying the content of two ram kits. Let me explain…

Some time ago I bought a used MB+CPU+RAM combo.

It’s actually the system I have listed in my profile (ASUS Sabertooth P67, i5 2500k, 16GB DDR3 1866 CMZ8GX3M2A1866C9).

But here’s the thing: as you can see from the SKU, that’s a 4*2GB kit, and the previous owner just bought two of them. I quickly realized that, despite the two kits being identical, they have a lot of trouble running together. The system eventually always crashes under stress.

I did some quick tests:

- tweaked (and eventually removed) the CPU OC. No change in stability whatsoever, it doesn’t look like it’s the CPU’s fault.

- tested those memories on another system that I have (Sabertooth 990FX, FX8350), same problems. For the record, this system is running just fine with a 2*8GB 1866 ram kit. So we can exclude CPU and motherboard.

- lowered ram frequency to 1600MHz, system still unstable.

- ran memtest 86+ for about 10 hrs. No errors, but when I try to run it in multithreaded mode it always locks up within minutes, and this happens both on the Intel and the AMD system (this may be a memtest 86+ fault, though. I didn’t test with other kits/systems).

For now I just found a spare 1600 kit I had lying around and I’m using that.

I know that I can still try to make them work by tweaking timings/voltage/frequency, but there’s something I don’t know: is there a way for me to know which of the sticks went together in which kit? Of course I didn’t buy this hardware new, and I don’t know how the previous owner mixed the 4 sticks. Is there a way to at least put the two kits back together so that I can either use them as 8GB or sell them and get a proper 16GB kit?

The only numbers on the stick that aren’t identical on all 4 of them is a 6 digit number that comes right after the size of the stick (see pic). Two of those numbers are consecutive, so I intuitively thought that those two sticks belong together, but I honestly have no idea, I don’t even know whether that number is actually a serial (or maybe manufacturing date?), or whether it’s relevant at all for my purposes.

Any advice? Thank you in advance!