We've never seen a shot glass (well, it's not really glass, it's made of aluminum) that adjusts from a shot to a jigger. It is quite clever and perhaps was meant for travel. Aluminum (or aluminium) was all the rage
It has been rumored that Napoleon had an aluminum rattle made for his child! In the 1940s, aluminum was a popular metal 1940s for World War II. Aircraft was built with a substantial amount of aluminum becuase of its lightweight characteristic. The 1950s introduced cookware, serving dishes and all kinds of accessories. We're guessing that this little shot glass hails from the 1950s. This type of item was also produced in the 1960s, but such were more rare.
The measurements are 1-1/2 inch diameter at the top and 1-3/8 at the bottom. Closed, it measures 1-1/2 inch high (for a shot) and 2-1/4 inches high (for a jigger).
Of course, you may use it as intended, or you might have an exciting new idea for it. We have not cleaned the shot/jigger glass, as we do not want to ruin the value of vintage or antique items. Aluminum items like this should not be washed in the dishwasher or with caustic cleaners: dish washing soap and water would be best.