Most factory Mustangs had holes drilled into the trunk lids for the placement of the M U S T A N G letters
and their pins on the back. Very often on replacement trunk lids, or by request of an owner to a body
shop, the holes were either not drilled or filled in and painted over. If your trunk lid does not have
the pin holes readily visible, there are replacement letters that are stick on that look just like the original letters.
Below is a diagram of the measurements to either drill the pin holes or at least mark
where the stick on letters should be placed on the 1969 and 1970 Mustangs.
I hope this information is helpful to you.

 
 
Diagram and dimensions courtesy of "B9GA" of the Boss 302 Registry