This is the first grill to be issued after the experimental A and C grills. It was produced in April 1868. The 1¢, 10¢ and 15¢ were printed near the end of the Z grill life span. They were printed for perhaps only a few weeks and as such they are rare, with only a few sheets of the 15¢ being printed.
Only two copies are known to survive. Both are in private hands. Here is the other 15¢ stamp.
The Z grill was discovered in 1913 by William L. Stevenson who created the grill letter classification system. At the time he could not match this grill with any other grills, one year later, because of its unknown nature he assigned the letter Z to this grill.