As I recall small inboards and magnetos, they were usually mounted to a quick fastening device, and were taken home and put in a warm safe place ready for the next outing. In an area where many owners ran the same engines, it was wise to match the magneto and engine numbers as protection against the less than honest folk who may just swap their sick maggie for your good one if you looked away for too long.
Many small river craft were part of hire fleets and you would be handed the correct maggie to attach before you sailed off. No need to lock the boat up in daytime,and many of the old clinker built tubs were too heavy to heave out of the water readily.
Just my recollection, Combustor.
