One version has the Angels name with a lower case "a"
and the other version has an upper case "A", along with a darker shade of red
The signs are sometimes referred to as being from 1973 and sometimes from 1974. Perhaps they were issued in both years. One clue is that there are different colored boxes that the signs came in, which perhaps indicates that the signs were issued twice:
Unfortunately there is no date on the bottom of the box to help confirm:
The boxes appear to be exactly the same, including the item #. It would make sense that the different colored boxes indicate that these were issued at different times, and help explain why they are identified as being from either 1973 or 1974.
In checking through old pictures of ebay listings I had saved, I did find a picture of a set with the small "a" with a blue box
I also found a picture of the set with a red box, but unfortunately the Angles card isn't showing:
Based on the information provided by Mike on the Angels variation, along with the different colored boxes, I believe the sets were issued in both 1973 (blue box) and 1974 (red box).
Thanks Mike for passing this information along and helping shed more light on this set.