Vintage Big Ben Alarm Clock Advertisement

My grandparents owned a jewelry store for about 30 years. About 12 years ago I was doing some remodeling for them and came across this vintage advertisement behind a wall. It looks like it is for the Big Ben alarm clock company. If you just look at the first picture it is like a plain picture but when you hold it up to a light a saying appears that reads"He left is call with big-ben". This is painted on what seems to be metal screen. It has the original frame and look like it might slid down over a light box of some sort because there are small metal rails attached to the back to slip down over something. I have looked everywhere for one like this to get more information on it but have not succeeded. Just wondering if anyone else has come across a picture/advertisement piece like this.