Rarer than the Fatboy we commonly see today, there was the Bottom Dial.
Gillette was searching for a razors that could adjust to the demands of many different kinds of men and beards. They tried multiple razor heads / handles with the Light | Regular | Heavy Super Speed line in the mid 1950's. But Gillette engineers knew that 1 razor that could adjust would be even better.
Many incarnations of adjustable razors came forth - some better than others. The original Bottom Dial Fatboy was a flawed razor mechanically, but extremely sought after today because of its scarceness and historical precedence.
These razors were likely more than a prototype - a limited release is a better description. Today there are a few hundred in the market and they command a high price from both collectors and shavers alike.
Below are some engineering drawings that really show the evolution of this razor. This is patent number 2,952,911 filed on March 19 1956 and issued on September 20 1960, a gap of four and a half years. The inventors are Meyer J Shnitzler and Michael Douglas Benedict Jr.
As you can see the adjuster in the drawings went from 1 to 5 and in the production Fat Boy, Toggle, Slim etc it went from 1 to 9.
Also the adjuster in the drawings is near the bottom of the handle just above the twist to open (TTO) knob whilst on production items it was moved to just under the head.
Gillette built a very small number of prototypes as per the patent drawing. These now are extremely valuable. There is one in the photograph below.
Another complication here is that Gillette actually had two different adjustment mechanisms. The one in the Toggle adjustable moved the guard bars.
The one in the Fat Boy bent the blade to differing degrees. This patent says: “This results flows from the fact that the relative positions of the guard member is controlled by the adjusting knob”. So it looks like it isn’t the Fat Boy mechanism.