Mercon Type F Automatic Transmission Fluids were the Ford specification for ATFs prior to 1977. Most Ford transmissions manufactured prior to this year as well as several Toyota transmissions manufactured in the 70s employ Type F ATFs - as long as these engines still employ the original clutch and band materials. These fluids are commonly used in C6 transmissions.
The main difference between these fluids and the later Mercon fluids is their absence of friction modifiers: this does not allow for any sort of slippage when shifting gears, making the shifting process very mechanical and resistant. Resistance in shifting is commonly desired result in performance applications. Additionally, these older transmission styles (though some post-1977 Ford transmissions have specified a Type F ATF) likely respond negatively to the friction modifier packages present in newer Mercon fluids and may generate excess heat and contribute to quicker fluid breakdown.
As is always the case with employing new lubricants to your engine or transmission, be sure that the manufacturer of your vehicle specifies a Mercon Type F ATF as the ideal transmission fluid for use.