Here is an update with some new information that I was able to compile and a nice selection of new images showing the use of the Speedmaster during EVA's by American astronauts up until its use was discontinued.
To update and answer the question of when was the last time an American astronaut wore a Speedmaster externally during an EVA, it was on September 16, 1993, by astronauts James Newman and Carl Walz during their STS-51 EVA.
Here is a chronological history of the EMU glove with the integrated Speedmaster covered by the flap. A few of the images reveal a peek at the Speedmaster but I have been told that if the glove had the flap a Speedmaster would have been present as standard equipment whether it is seen or not.
Picking up with the first Shuttle EVA during STS-6...
1000 Series EMU Baseline Glove. This glove can be identified by the tan canvas palm. There was a later 2000 Series Modified EMU Baseline Glove that did not fly.
STS-6 - April 7, 1983
The following are images in chronological order using the 1000 Series Glove. The flap seen on the right glove indicates a Speedmaster was in use.
STS-41B - February 7, 1984
STS-41C - April 8, 1984
STS-41G - October 11, 1984
STS-51A - November 12, 1984
STS-51D - April 16, 1985
STS-51D with watch edge visible.
STS-51I - August 31, 1985
Introduction of the 3000 Series EMU Glove an evolution of the 2000 Series. This glove can be identified by the gray rubber palm. There was also a 4000 series but it's use is questionable and was introduced during the delay period following the Challenger accident.
STS-61B - November 29, 1985
Introduction of the 5000 Series EMU Glove and evolution of the 3000/4000 series. Flight version of the Phase IV High Pressure Glove
STS-37 - April 7, 1991
Introduction of the 4750 Series EMU Glove and evolution of the 4000 Series w/ 5000 series TMG (Thermal Micrometeoride Garment)
STS-49 - May 10, 1992
STS-49 with watch edge visible.
STS-54 - January 17, 1993
STS-57 - June 25, 1993
STS-51 - September 16, 1993
Astronauts James Newman and Carl Walz during their EVA, the last to wear the 4750 Series Glove with the integrated Speedmaster September 16, 1993.
Introduction of the Phase VI current EVA Glove variant no longer carrying the intergrated Speedmaster and its use during EVA's by American astronauts is discontinued.
STS-61 - December 5, 1993
There you have it. More than you ever wanted to or needed know about the subject of EVA gloves.