Like everything in sailboats, it depends...
The two major makers of masthead lights are Perko & Aquasignal (that I know of). Oddly enough, they tend to use at least two different bulbs. These are "festoon" bulbs, the longish ones with the contacts at the end. Perko bulbs have a point at the end contacts while Aquasignal have a depression at the end contacts. If you are lucky, it's one of these two types. buy one of each and go on up!
