I love these instruments @NickA, thanks for showing us yours.
There are 7 different Viole di Gambe, literally Legs Violins, but their tuning is quite consistent as it's fourth fourth fourth third fourth fourth and the clef goes from F to G passing by (the 3 different) Ut (C) clef.
The higher register model can sometimes only have 5 strings and the bass register model can sometimes have a 7th string, often tuned lower.
They were only partly fretted, because there was no use of higher registers at the time (XV's century !!!) for a solo instrument and with 7 different sizes covering all the available register, these instruments were often used as a whole "orchestra", or played within a "chamber orchestra", especially in the beginning, or played solo without all the modern stunts.
Try to tune it and play it, and you can use a modern tuning in fourth if it's easier as there's no strict rule about it. 😉