Could someone help me identifying which of the three closed belts in the Eustathios BOM is for the Z axis? Thanks.

I can help you with some maths and a process of elimination: I assume you can measure the distance between the center of the x/y shafts and the corresponding motor shaft. Once you have this value for both X and Y axis, multiply it by 2 and add 64mm (pitch circumference for 32tooth GT@ pulley). That should give you the x and y belt lengths. The lengths on the BOM are 502mm, 497mm and 475mm.

It’s the 502mm one (the longest one)