Hi Ben, that was a good bargain, these are very nice carburettors, my Citroen 5HP has a similar one. Also with barrel throttle. The venturis are just bolted in so you change them to the right size.
This belgian collector has a lot of old parts for sale as he is selling the lot of his collection. He only speaks french but Google translate is a good help.
http://levidegarage.wifeo.com/
Regards Fer