This (#410) and some other buses of Pletschette have a small number written on the corner. Could it be some kind of fleet / inventory number used by the operator or a number assigned by external agencies (like AVL or RGTR)?

Other number examples: https://fotobus.me/photo/3914328/ #2, https://fotobus.me/photo/3913822/ #403 or 405, https://fotobus.me/photo/3913120/ #408, https://fotobus.me/photo/3913121/ #409

These numbers aree intern inventory numbers. But I do not have a comprehensive list.
There were on older buses 1-digit numbers, at a later momen 3-digit numbers in the 400 range.

Alright, will add those numbers to the database - a few numbers is still better than nothing :)