Space Cover #57, Two on One
Designated prime recovery ships were generally US Navy aircraft carriers. With ships of this size the post offices aboard the ship used a machine cancel to postmark the mail from the ship. For the Gemini flights the most readily available cancels for the USS Intrepid, USS Wasp and USS Lake Champlain for their recoveries are machine cancel covers though covers with hand cancels can be found as well.
For Gemini 8 the USS Boxer was designated as the primary recovery ship but an emergency landing turned the USS Leonard F. Mason from a secondary recovery ship into the actual prime recovery ship. The USS Boxer used a machine cancel on the majority of the covers sent to the ship with hand cancel covers created as well.
The cover shown is a very rare cover indeed as it contains BOTH a machine cancel and a hand cancel. It will not be known why both types of cancels were used - maybe it was because the machine cancel is somewhat hard to read or that the second row of Fort Bliss stamps was not stuck by the machine postmark. How many such covers exist, if any others, will also not be known but this was the first one I have ever seen for this mission and only the second such prime recovery ship cover I've seen with both machine and hand cancel types from the ship for the recovery date. So if you find one...