Also, does it have a Selenium or CdS (or other) cell? (i.e. doe it have a battery? - if not, it's selenium, which usually covers a fairly wide area, dependent on any microlens arrangent placed in fron of the cell).
Best would be to do some creative experimentation, trying to work out just what the meter is reading - a good idea anyway, even if you already know what the theoretical coverage is.