Hi Kevin,
It has to do with the top (the head of the screw).
Type 1 is a normal phillips
Type 1a is "Star" Phillips (it has extra cuts for a smaller phillips screw driver)
Here is a link for it: look for the Drives Section (PH and PZ are the other reference names for them)
http://www.pencomsf.com/screws-categories/screw-design-guide/
I hope this helps,
James
One small correction for the Type 1A:
The Smaller cuts are not for smaller phillips (although I'm 100% guilty of using a smaller screwdriver on them...LOL)
Its for the special Star cut Phillips (it gives better torque, and appearently has less chance of striping)
Regards,
James
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