I own a copy of the following books:
"Pierre Levasseur et ses avions " TMA 2002;
"Les avions de Pierre Levasseur" LELA PRESSE 2004.
There are two interesting photographs inside which reveal a little of the cap that covers the water reservoir at the head of the cylinders.
I'll try to scan them tomorrow at work. For what it is worth. From what I've been able to measure, the cap is about 8 cm in these photos, and the pipe it covers about 6 cm.