Hi:
I've only found a few references but it is a start:
This is in relation to St. Goar and the engineers crossing the Rhine. "The 168th Engineer Combat Bn was to support all assault crossings, the 188th Engineer Combat Bn to construct rafts an take charge of all ferry equipment, and the 243rd Engineer Combat Bn to build a treadway bridge at St. Goar."
"243rd Engineers launch a Bailey Bridge across a gap in Italy, 1944. IT was painted y Ludwig Mactarian a world war II artist/correspondent serving with the 337th Engineers, Fifth Army. An incredible 200 miles of fixed Bailey Bridge was manufactured for WWII."
I have a photo in one of my engineer books that shows some engineers at St. Goar connecting bridge sections. I will scan it later and send it to you. Sorry I can't do it this morning, but have to get ready for work.
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I suggest that you contact the Army Corps of Engineers who were an immense help to me when I began my search. They keep folders on each unit. Some units have much for info than others. Hopefully they will have something to get your started.
(Updated link - January 2012)
http://www.usace.army.mil/Contact.aspx
Please visit my forum here to find other research tools. To save you some time, I have already searched the association contacts and there are no listings for the 243rd. If you go to the WWII Connections site and do contact Richard Horrell, please tell him that I sent you. He is a friend of mine and fellow colleague.
http://www.6thcorpscombatengineers.com/engforum/index.php?showtopic=23
Let me know what else you find out. I will post your letter and my answer to you in the forum, so all can see it. You never know. You may also want to sign our guestbook so people can also see your quest there.