Hi Ken:
First off, it does stand for Distinctive Insignia. Correct!
The following is scanned from my American Society of Military Insignia Collectors book, Engineer's edition. Volume 1, December 1996.
The entry in the book refers to the 393rd Engineer Bn.
Now there can be variations, but insignia were NOT shared with other units. Some may be similar, as is the case with the 36th and 540th, as they shared history. In fact, the 540th 3rd Bn actually joined the 36th, and the 540th remained as a two battalion unit throughout the war. Their seahorse insignia bear a close resemblance.
Just for further clarification and reference, I am going to join this post with an older post on the 393rd.
Back in a moment after I do this.
Thanks for replying. I meant the DI comment in a sarcastic way 
I've collected WW2 memorbillia for almost 30 years, and never had a situation like this come up. But I'm also wise enough to know that there have been lots of stranger things to occur in how the Army badged itself. So, here are some visual aids I have to share.
First photo is the cover of the 393rd book I have. Note that I've only seen one other turn up on ebay in the past 10 years.
This copy belonged to my grandfather.
http://campclaiborne.org/images/393rd-SS/3...cover.small.jpg
This is the first page of the book, and explains the details of the 'blazon'.
http://campclaiborne.org/images/393rd-SS/3...ook-1.small.jpg
The photo you provided is the same as the DI I have. Well 3 of them. the other two are similar but different.
Next, is the 5 different DI's I have all named to the 393rd.
http://campclaiborne.org/images/393rd-SS/393rd-di.jpg
The top 3 are all the same design, no hallmarks and are pinback. Front and back features are consistent with it being the same manufacturer.
The lower two, lack the motto, the pelicans wings are not spread, and instead of white there is a silver bar. Both have no hallmarks as well, and the one on the left is pinback, and the one on the right is clutchback. Also note the pelican on both lower ones is a seperate piece, and not embedded.
In each case, they listed them as 393rd Engineer or Engineer BN. One might have been named Special Services, but I can't remember.
You can see why this is confusing me I have solid proof telling me one thing in the form of the book, newspaper acct and photographs. Things I can hold in my hand. Yet something totally different keeps turning up. Then there is the 2 oddball DI's that I don't know what to make of.
Being able to see the 393rd General Services DI(if they had one, not all units did) would at least help me narrow the playing field. My working theory(for lack of anything better) is the motto lacking DI's are the GS regiment and the SS regiment had the one with the motto and flying pelican.
It's just a theory based on frustration, but who knows.
OH, I almost forgot the kicker...... In the whole of the 393rd book, and all other photographs, I don't have a single solitary image with a GI wearing one. I have group photos of the whole Regiment, man after man in Class A's(including my very proud buck sergeant of a grandad) and not a single swinging **** has one.
So, makes it even more interesting for me.
One of these days I'm going to get it sorted out.
Ken