Net_Stamp,
Your stamp, C12, was printed on a flat plat press in press-sheets of 200, and cut along the inked guidelines into panes of 50 for retail sale. Your stamp is identical to those in the left-hand column of the pane below and contains a natural straight edge and part of the guide line. Very common. Borrowed from
ebay:

In contrast, C16 was printed on a rotary press also in press-sheets of 200 cut into panes of 50 for retail sale. One obvious visual difference is the selvage on all 4 sides of these rotary-printed pane, thus all C16 stamps are fully perforated. Borrowed from
ebay:
