I stand corrected. the 223rd Engineer Combat Company is the correct unit. I found one mention of them in the book, Technical Services, Corps of Engineers, the War Against Japan by Karl C. Dod, page 332-333, The 223rd & 521st Engineer Combat Companies arrived on Kiska Island about Sept 1943 and took over base construction from the departing 2nd Bn 50th Engineers.
There is no other mention of the 223rd in the book.
The 223rd has no campaign credit for the Aluetian Campaign, but the 521st does. Credit for the Aluetian Islands Campaign covered the the period: 3 June 1942 – 15 August 1943.
At the moment i cant find my list of Army Apo`s, but from other sourses it appears APO 726 is Attu Island. Mail from Alsaka would have been sent to Seattle for routing to the address in the US.
As for the Report of Separation, there is about 30 more boxes of information on it. The parts your missing contain enlistment information, awards & decorations, campaign participation, dates of travel outside continental US, etc. I suggest you contact the County Clerks office & see if they can provide the whole page, they should have it.