Platoon(s) are unit(s) of men inside a Company, A Company or Companies are inside a Battalion. A Battalion(s) are inside a Regiment or Brigade, A Reg/Bri are inside a Division. Divisions make up the Corps. So there can be more than one 745th Platoon in several Companies, inside several Reg/Bri, inside several Divisions inside the Corps.
Ancestry has U.S. Marine Corps Muster Rolls 1798-1958 and 7 entries for Jack E. Felker. They are scanned images of typedOfficers and Enlisted listed (Mustered In) of Units for a Quater (4 months) periods of time or a month length of time. There are service numbers, Remarks of transfer, MOS (job), unit and place stationed at time.
First entry for Jack ....Month of Oct 1943 (Oct 1 to Oct 31) Muster Roll for4th Recruit Battalion, Training Regiment, Recruit Depot. MCB San Diego, CA (MCB= Marine Corps Barracks) Felker, Jack E, Pvt, enlisted Oct 12, 1943 Remarks...(blank)
Last Entry...July 1945 (July 1 to July 31) Muster Roll (listed) in Amp Trac Bn, FMF, Pacific. (Amphibious Tractor Battalion, Fleet Marine Force) Crossed out with a typed line is C/O FPO San Francisco. #58 (list men alphabetically and by Rank grouping) Private First Class Felter, Jack E.Remarks...996 Co. "A"
Another Muster Roll has remarks "See Footnote C" and "Carpenter" and I don't know where the footnotes are in the scanned images. I don't have the time to look. I suggest you see if local library has Ancestry to use or subscribe yourself to print out pages and look for footnotes. There is a Fold3 subscription site that supposedly has more Military databases than Ancestry. They partner with LDS FamilySearch.org. You can find results through FamilySearch but you will have to subscribe to Fold3 to see images.
The NARA has WWII Marine Corps Service Records. They were not stored there when the fire happened in 1973. http://www.archives.gov/veterans/http://www.archives.gov/veterans/ Be prepared to provide a death certificate as copies of records only available to next of kin after death of Veteran.
Probably has more information than online Military databases released so far.
He should have a "Victory" Medal given to all who served in WWII, he should have a "Pacific Theater" Medal since it looks like he served time in the Pacific theater of war.
Here is a link to the names of medals awarded WWII. http://en.wikipedia.org/wiki/Military_awards_of_World_War_II#United_Stateshttp://en.wikipedia.org/wiki/Military_awards_of_World_War_II#United_States or do an internet search of the names on the medals or "WWII Medals" you will find mostly sites the sell them but you also get good pictures and explanations. There are also medals and decals specific to the branches of services. So do a search U.S. Marine Corps WWII Medals and Citations.
There is a book written by someone about the 5th Amphibious Tractor Battalion that served in WWII. There were other Amp Trac Bns, all seem to have been called "Alligator Marines".