Charles is right...
The Pony Keys were the only keys that the factory gave out.. when a '64 1/2 to '66 Mustang car was purchased new.
People love the Ford Factory Generic Keys though (as do I), but if you look on the key, It has a "V" on it meaning "V-8"... I think that they mostly came with Early 60's T-Birds and some Full Size Fords when new...as mid-size cars also came with sixes...such as the Falcon and the Fairlane...so chances are that they would not have been supplied with the Generic Ford Key pictured up top.
On actual Ford Factory Made keys, They have letters stamped on them... "B", "H", "I" and other letters? The letter on the key denotes the key maker of the key. I know that "B" is Briggs and Stratton, and "I" is Ilco, and "H"is Helm? A lot of the "I" (Ilco) Pony key blanks came from NPD from the past two decades. They are also Original Ford Key blanks...(most likely Ford Factory Replacements) Rick Schmidt and his father from NPD had an abundance of them up until the last few years as their supply dried up. I have noticed that Original 1960's Assembly line Mustang Pony Keys and Key blanks were mostly the "H" (Helm?) stamped keys.
Basically, You will know that you have a factory key when there is a Letter on it. No Letter, Then it's not a Factory key. Also, Most Factory keys will usually have "USA" stamped on the back, but not always...but most of the time. If you have both a Letter and "USA" on the key, then it's a sure winner that it's an original Key or Key blank.
About the Reproduction Pony Key Blanks....Most of them are really bad... and also Dennis Carpenter makes/sells Repops of the Ford Generic Keys that are made in China... They are just as bad...a big disappointment... Basically, If you are looking for Original Blanks.... Your only real source is Ebay... Definitely a situation where there is no substitute for the Originals....IMHO...
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Tony K.