The Stanley Adjustable Bit Gauge which is also often shortened to as the 'Stanley Bit Gauge' was a Stanley tool used in conjunction with auger bits / drill bits used in hand drills / hand braces.
The Stanley Bit Gauge is attached to a drill bit and is used as a depth stop allowing hand drilling to a specific depth.
The gauge can be set to any depth within the limits of the drill bit / auger bit.
The Stanley 49 Bit Gauge was advertised as being able to be used on auger bits up to 1" (25mm) in diameter.
The Stanley 49 was 2 1/2" (64mm) long, made from cast iron with a Nickel plating finish.
Last image is one that I found on the internet for reference only.