No F5's during Palm Sunday Outbreak of 1965?

I watched a documentary about the tornado outbreak on April 11th, 1965 tragedy and I was surprised that there were no tornadoes labelled F5. I understand that this was before the Fujita scale was even introduced, but considering how some of these storms were able to produce tornadoes at will, including multiple storms having multiple twisters at the same time, some of these had to be F5 level, no? The environmental instability had to be immense.