That does look to me like a shotgun style trigger guard, and I would really expect to see that there was a screw at the forward tip of it which held it secure to the front of the trigger plate.
Is that a bolt head, at the rear of the magazine well?
Based on what I see there I figure the trigger guard front strap has the screw thread on it and dried mung is keeping the whole from turning. My first thought would be to apply some form of padding (leather, or rolled up layers of paper towel, for instance), get a secure grip on it with a clamp or vise grip type pliers, and gently (!!!!) apply pressure in both tightening and loosening directions. I would clamp on so as to best avoid having the front strap and the front of the trigger guard move relative to each other as that was tried. Gentle!
Material? Celluloid, patterned to look like Tortoise Shell is another possibility. Dig around the web and you will find the description of a hot pin test that will help you sort out the material if you wish to. Essentially you get a pin red hot and touch it to a not visible portion of the material being tested, and observe the reaction, whether it melts or burns, and smell the smoke, for plastic or burnt hair smell.