As far as I know, all early Mustangs with an alternator had a pigtail in the harness for an RFI condenser at the pigtail for the voltage regulator. This was to "supress" the static from the regulator points opening and shutting. RFI was a shortcut buzz word af that time for Radio Frequency Interference. In 1965 there was the very visible condenser mounted externally on the alternator and was there to supress RFI caused by the alternator. Remember, alternators were at that time about equal to multiplexing of electrical circuits in todays world. The condenser on the alternator is identified by the large mounting tab and on the end opposite of the pigtail it has the C5TA-18827-A Ford engineering number stamped on it. Look at an original one on the closed end and you will see this engineering number plainly embossed. In 1966 the Mustangs still had the RFI condenser located at the voltage regulator location, still had the red tag to identify what the pigtail was, and as far as I know used the same condenser. However, in 66 the condenser for the alternator RFI was moved INSIDE of the alternator. You must take the alteernator apart to find it, and then it will be difficult to recognize as a a condenser, but it is there. So nothing really changed much, but the obvious condenser on the 65 Mustangs was relocated within the alternator. And you are all correct when you say the 65 external condensers are difficult to find. As an aside, it is possible to replate them with no damage to the electrical function. I have had several done in zinc plating along with other odds and ends and they have continued to work for years and looked as new. I am pretty sure they have no function other than supressing the alternator "whine" in the AM radio when listening to a distant station and that the charging system will be happy with or without the condenser. T sum up this rather lengthy explination, the 65 production with alternators AND WITH A RADIO had two external and visible condensors, one at the voltage regulator and one mounted on the alternator. The 66 production cars WITH A RADIO had only one visible condenser, and it was mounted adjacent to the voltage regulator. Both the 65 and 66 Mustangs had the red tag looped around the wire harness pigtail to identify it as being used for a radio interference supression condenser. Cars without a radio DID NOT use this wire harness pigtail for anything - it just hung there.
Brant, I sent you a couple of images of the details on the 65 RFI condenser mounted to the alternator with the FORD engineering numbers shown in detail. Post these if you think others would be interested in how they were mounted and how they were identified by Ford in the day.