I have decided to replace all of our 25 year old nav lightbulbs with LED's. While searching over the weekend, I found a company called MastLight that has exactly the replacement we need for the 31mm festoon lamps in our Perko masthead fixture. These are relatively small bulbs and although Ancor sells replacement LED's they are "not intended for use in navigation lights". The MastLight replacements use a single high output LED, the Ancor uses a four LED array. Their website actually uses the same fixture that we have as an example for their product.