The 10 cent & 25 cent 1st verses 2nd printing are a very noticeable difference in color shade.
However the 5c cent black & gray with red lettering is far more difficult. I just discovered a possible way, but not fool proof method. Using a short wave UV tag light, the 1st printing paper all shines rather brightish white. It has a blotchy appearance too. The 2nd printing shows a duller creamy buff color. I did find a few 2nd issues that were the whiter paper. My best explanation would be there was some paper left over paper from the 1st printing used up on the 2nd run but a majority of the 2nd run appears to have the more buff color paper. Please let me know if this is of help.
Another difference I have noticed is the accuracy of the perforations are much better on the 2nd printing. It is rather difficult to find VF centering on the 1st printings.
Also, not make what light you shine on 6 triangles put together, they make a hexagon