Comparing it with the picture of the C-2 that is in my 2nd book I agree with Charley. These visible attachments were sold by a number of companies. In the early 20s visible pumps were sold using advertisements that "now your customers can see that they are getting what they paid for." So, if you couldn't afford a new visible pump you could purchase one of these and attach it to the existing curb pump you owned. A number of them are shown in my book. Here is one showing a C-3 attached to a Bowser 101.

Jack Sim
Author, Gas Pump Identfication books 1st & 2nd Editions & Air Meter Identification books
We rebuild ECO air meters and sell parts.