Chieftain L15A3 APDS Flat Penetration

In game the round has lost over 100mm of flat penetration over time and I am wondering what sources can be found regarding the flat penetration of the round. I know of a German report that cited the 320mm that existed before the latest update, but clearly something has overwritten that.

I found a 1962 report on the Chieftains gun but it seems the entire topic was regarding angled penetration.

Here is yet more evidence that HESH is still not implemented properly.

And another graph showcasing that the round could pen 150mm at 60o up to 1600m