The photo would seem to have been taken before the frame was powder coated so I think the frame number can just be seen running down in a vertical line directly below the upper bolt hole in the gusset plate.
Above the frame number you should also find a single number '4', that is the model code number for the ES2.
Was the ES2 a model name then or were there a number of different models which went under this generic name?
'ES2' was a specific model name although the specification did change quite a bit over the years as it was in production from 1928 to 1939, then 1947 to 1963, also (ES2 Mk2, 1964 to 1966).
The frame number will give the year if you can find it and say what it is, the first three digits of the number reading top to bottom should do (do not include the separate number at the top)?