Are we talking GM as in General Motors, or something else? If you don't know the original use of the sign, could you at least tell us what you want to do with it? Where did you find the pic you posted?
Yes we are talking about GM as General Motors and this sign was on locomotives in early fifties. I need a better copy to make (or draw) a new sign (decal) G12 model from GM in H0 scale.
Have you tried the Fallen Flags site? they have lots of pictures of locos. Maybe you can find something useful there. In addition to pictures organized by railroad name, I believe there might be some demonstrator and/or manufacturer pictures too.
Have you looked at the Microscale site? www.microscale.com. I just checked their online catalog and they have a bunch of sheets for EMD demonstrators. If they have the decal you need, the prices are reasonable enough that I don't know if you would want to go to the trouble of making your own.
if anyone watched barret jackson, they know what this is... the thing has a nice big GM logo on the front, not quite what you were looking for, but it's of the style and era
Here is the logo - not too sharp, but at least it's a start from here. I think you can use it as a base for souping it up a bit with Photoshop or another graphics program.