It looks like a predecessor to the wearable coms that showed up in TNG. These crests were worn on officers uniforms all over the place in the early trek films. I'm thinking this may be a rank indicator, since it seems to have a small pip in the upper right hand corner of it.
It's made of plated cast metal, and has fasteners on the back.
Phaser1 tells me:
The piece on your website is not a Lincoln Ent piece but a Starland, Intergalactic or other source piece..the original piece is arced, not a flat piece
And David Simonds tells me:
that beautiful FED Crest is in fact, the Federation Insignia worn on all uniforms regardless of rank. The little "dot" is supposedly a communications device (like the button on a mic) The badge was worn on the Left chest of the Red jacket. The rank pins were a bit different and were worn on the opposite shoulder strap.
For the final word, J.L. Garner, III tells me:
From The Wrath of Khan up until TNG invented the "combadges", everyone in Starfleet had the same basic insignia: the delta shape with a circle behind it. The gold-colored oblong piece behind the pin was only worn by officers, and the pip was just there; it doesn't really mean anything. The actual rank insignia were the pins worn on the shoulder strap.
Thanks for the info, guys.