Mackinaw Jeep Coats were issued by the US Army and US Army Air Forces during WW2 and were meant to be worn over the older M41 field jacket during cold winter weather. They had an OD3 cotton poplin shell for wind protection and a “blanket wool” liner for insulation. Like the M41 jacket, these were supposed to be phased out by the end of the war since the more modern M1943 series uniforms were being introduced, in a much darker OD7 color and without the waist belt, but many older Mackinaws were still photographed in use by the end of WW2 (sources: https://www.usmilitariaforum.com/forums/index.php?/topic/342244-mackinaw-jacket-question/, https://www.usww2uniforms.com/252A.html). The “jeep coat” received its name due to the 3/4 length, a result of a pre-WW2 US Army study on how to optimize uniforms for mechanized warfare. The shorter length compared to the pre-war full length overcoat would have allowed personnel to more easily and quickly maneuver and dismount from vehicles (source: https://www.usarmydatadepot.com/55_c_33090_a_55_c_33190_mackinaw_6347.htm).
Patches:
Ruptured duck indicates the previous owner was honorably discharged at the end of WW2 (source: https://www.tracesofwar.com/awards/2199/Honorable-Service-Lapel-Button—Honorable-Discharge-Emblem.htm). 
US Army Air Forces patch. 
USAAF patch is hand-stitched to the outer shell layer.
Tag:
This particular jacket was probably among the last of its type (Type I/Spec 6-104A) produced as seen by the June 1942 production date (source: https://www.usarmydatadepot.com/55_c_33090_a_55_c_33190_mackinaw_6347.htm)
Tag is located on reverse (liner facing) side of pocket. It would not be visible inside the pocket.
Collar:
External wool collar indicates that this jacket is the earlier “Type I” model that was produced until the second model with a plain poplin collar was adopted on August 27, 1942 (source: https://www.usarmydatadepot.com/55_c_33090_a_55_c_33190_mackinaw_6347.htm)
Pockets:
Buckle:
Even the belt has insulation material on it. 
The belt was removed in the final version of the Mackinaw jeep coat (source: https://www.usww2uniforms.com/252A.html).
Cuffs:
Usage Photos:
December 22, 1944: American soldiers captured by the German Wehrmacht during the Ardennes Offensive (Battle of the Bulge) (from https://commons.wikimedia.org/wiki/File:Bundesarchiv_Bild_183-J28589,_Kriegsgefangene_amerikanische_Soldaten.jpg). 
January 1945: US Soldier inspecting a knocked out German Panther near Kelberg, Germany (date from https://www.usarmydatadepot.com/55_c_33090_a_55_c_33190_mackinaw_6347.htm, photo from https://www.worldwarphotos.info/gallery/germany/tanks-2-3/panther-tank/panther-tank-kelberg-germany-1945/).
