Some thoughts on the question PJW asked about availability of records.
I`d be willing to bet that the histories & operations reports of all The Engineer Combat Groups are in the the National Archives in Washington..... so why arent they available? It may be a case of numbers.
First someone has to make them available. It takes time and money to get the records out of the archives. A veteren of the unit, or a son or daughter or grandchild has to have a desire & resources to research, build a website, etc.
An ECG was basicaly a Headquarters & Headquarters Company consisting of 15 officers, and 69 enlisted men probably with little turnover rate of personal. So you have maybe 100 veterens & using the the standard 2.5 children per household, thats a pool of around 300 people with an interest in that group. now compare that to an Engineer Battalion of about 630 officers & men giving a pool of around 16,000 people interested , or an armour or infantry division of 10 - 15,000 men with a high turnover rate the pool could be 100,000 people.
So you can see the number of people from which that 1 person who will get that info out is extremely small for the ECGs & the same holds true for other small "rear echelon" units.
I havent checked how many members of this forum have joined looking specificaly for info on on a ECG but it seems to me that the number is relatively small.
Other sources that provide online documents & reports give a higher priorty to the combat units like M1 said.