Yes, thats right. Excelsior were but one of the manufacturers who used the watercooled Villiers 250 engine in their range. I have the relevant sales catalogue somewhere.
Off the top of my head SOS, Excelsior and Vincent all spring to mind as users. I'd like any one of them but I confess the thought of turning up at a Vincent rally on a Villiers powered Vincent is high on my bucket list.
I've been told Vincent made half a dozen Villiers powered machines. The first had the standard aircooled 250 fitted but they were unhappy with it. In his autobiography Phil Irving describes how they found this engine didn't have the required stamina and when ridden hard would go off song but the watercooled version did not and was a tireless performer. Times were hard, they were desperate for sales so they made a batch to keep the factory ticking over. Times improved so no more were made.
Cheers,