I'm willing to bet Esci on that M1, as far as original producer.
When applying a clearcoat over decals, you have to be sure of using a "cold" clear that won't eat the decals. Usually I use acrylic clear (Model Master, personally) over decals and have no problems. As far as your polishing concerns go, skip the two coarsest grits in your polishing kit on the first coat and lay down about three coats. That will net you the best results.