There's no length specs for the real dipsticks, there's just the "correct one for a specific engine". The reason they have a real dipstick is because they don't have the digital display system in the vehicles that use that engine. The measurements out there are the settings for the T40178 calibration tool, for those engines in vehicles with the digital display system. Once the distance between the two scales on the T40178 was known, it's just math from there.
As I mentioned earlier, the AB stick is for the EA896 3.0 TDI. The MY08 A5/A4 were the first vehicles to implement the digital display, so we have both a stick and T40178 values for those engines. At 32,16,0, then we knew the AB stick was going to be 518mm top to full, and 16mm full to min.
The AM stick is for the EA897 3.0 TDI (CNRB) in the Q7. There are no T40178 values for that. So it turns out to be around 490mm top to full. Depending on the full to min distance, they could use that stick in the B9 Gen3 2.0T (CYMC, DPBA), which has T40178 values for 490mm top to full and 16mm full to min.
But for the EA897 3.0 TDI (CPNB) in the Q5, A6, A7, A8 in the mid 2010s, the numbers are 467mm and 12mm. No idea what physical stick out there might measure to that.
Though the CNRB and CPNB are both the same family engine, they are different. The CNRB uses 8.1 qt of oil, while the CPNB uses 6.8 qt, e.g..
It will probably be easiest to just get a T40178 knock off from Amazon and keep that in the vehicle. You can't keep it in the dipstick tube, but you can still put it in the spare tire well.
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