The Nautilus 400 series were round cased models in 10K Gold Filled cases.

The first series of photographs below is of the Nautilus 403. As you can see, this is a pocket watch but contains the normal 505 movement and was released in 1965. However, it sold very poorly; those people in 1965 who still wanted a pocket watch, rather than wristwatch, were not the sort of people who wanted a new fangled electric watch. As a result, the 403 is very scarce and one of the hardest Hamilton Electrics to find.

My 403 started as a case only; I added a 505 movement and was kindly given a spare dial from a Nautilus 602 which is similar to the dial from a 403.

Nautilus 403:

The following photograph show the Nautilus 403 alongside the Vantage electric pocket watch, an even rarer item!

Nautilus 404:

Nautilus 450: although the case is stainless steel, this 450 has a rather nice 10K gold filled bezel.