Space Cover #26, Tracking Skylab
In addition to the land based tracking stations, Skylab had a permanent naval tracking presence by the USNS Vanguard stationed off the coast of Argentina near Mar Del Plata. The USNS Vanguard was stationed there for the launch of Skylab laboratory and on-station throughout all three of the manned Skylab missions. There are covers such as the one shown that have a cancel from Mar Del Plata when the envelope has stamps of Argentina on them.
When US postage was applied, covers have a Cape Canaveral cancel (launch and splashdown dates) or an unusual non-USPS "cancel" used on the cover as shown on the top cover. The cancel is undated and gives the USNS Vanguard FPO ddress, "Skylab mission" and Mar Del Plata location. Covers with this "cancel" are common to find in collections.
Little is known about this "cancel" other than it was applied aboard the ship and, in general, was applied to covers with US postage. As a USNS ship, the Vanguard did not have its own postal cancel but relied on regular postal facilities for processing mail that was handled by the ships' Master.
I have seen the undated "cancel" on covers for all of the manned missions but not for the Skylab laboratory launch. Because it acted like an actual cancel on the stamp there is usually no other USPS cancel applied to the cover.