According to a few reliable sources. Apperently a few (100) American Shermans were converted by the British with 80 being issued equally to the First and Ninth Armies. The other 20 conversions were given back to the British. U.S. conversions appeared to have been called, Sherman M4 and M4A3 17 Pounder. There is no mention of Firefly. For obvious reasons it appears that these conversions saw extremely limited combat use if any. These conversions were only issued to U.S. forces in Italy 1945. I have come accross a Report of Action during period 11 April 1945 to 2 May 1945, Issued by Headquarters 755th Tank Battalion APO 464 U.S. Army on the Po River Campaign. There is the mention on 27 April 1945 of Company D making use of a single attached 17pdr.
Does anyone out there have any information or know of a source that may provide additional information of the U.S. Armies use of converted Shermans armed with 17pdr's ? .... If so it's possible we might be able to include these types to the U.S. OOB in a future update 1.04 of Matrix edition.
Sources:
http://frespace.virgin.net/shermanic.firefly/usnew.html and
http://755ththtank.com/porivercamp.html