I can provide the following related to the mounting hardware used for a 66 NJ built Mustang from my collection. I see a pattern but the sample is limited and since the cars were not assembled in sequential order the grouping should be a bit "fuzzy"
Hope it helps in some way
Pan head Phillips
6T10236x
6T11185x
6T11441x
6T11848x
6T12xxxx
6T134090
6T13772x
6T14212x
6T14378x
Smooth large flat head Phillips
6T14603x
6T150827
6T15534x
6T155639
6T15845x
6T15953x
6T16013x
6T1619xx
6T16125x
6T17172x
6T174634
Smooth large flat head Phillips
6T19415x
6T20186x
6T202378
6T20545x
6T20872x
6T20951x
6T215113
6T22139x
6T22352x
6T232546
Pan head Phillips
6T23421x
6T24357x
6T24594x
6T25110x
6T2601xx
Smooth large flat head Phillips
6T26174x
6T27875x
6T28533x
6T28607x
Updated through post 5/21/16 #38