About guns

Here is a phrase that I can't understand completely. A Russian soldier speaks about what happened in his front:
В начале сентября на нас двинулись немецкие танки штук 40.
At the beginning of september German tanks, about 40 pieces, moved towards us.
штук: gen. pl. because of 40 and "about" because of changing the order.
I don't know if it is correct. What do you think about it? Tks

You understood right.

Generally, you don't need to translate the word 'штук'. It means 'pieces' or 'items'.


'At the beginning of September about 40 German tanks advanced on us' is a good translation.