I know this thread is old but I felt for future reference this would help.
There were 121,000 mustangs made between March 9 and August 31. These are the 1964 1/2's. There were many changes from March until August so many of these are pretty unique having some of the 1964 1/2 features(battery fins, Master cylinder switch, non folded hood, etc) and some of the final(1965) features. Not all 1964 1/2's have all of the features uniques to 1964 1/2's. For example a 1964 1/2 made in March would have the radio that has the channel 5 as the lowest displayed channel, but the 1964 1/2 built in July would have a radio that starts with a 6 as the lowest channel.
Only 4999 1964 1/2's were built at the San Jose factory and carry the R VIN code.
You can't tell a "true" 1964 1/2 by looking for all the special features, but you can tel by looking at the door code date. If it was built between March and August it is a 1964 1/2. Also the serial number can help since there were only 121,000 built between that time. If the series number(last 6 digits of the VIN) are 221,000 or less it's probably a 1964 1/2.
At this time the 1964 1/2 is worth about $1,000 more than a 1965.