Spectacular Stirling Fountain from oh so long ago. Wonderful!!!
It just might be that its incredible condition is owed to the angled stream of water, making it less than a joy to operate. I would think an escutcheon pin, or other fine cylindrical object of suitable diameter, might be able to tweak the nozzle slightly, and straighten up the stream. Certainly opening the nozzle slightly would lower the height to which it squirts, and might help control it some that way as well.
What a wonderful little time capsule that box contains!