Alphonse Mucha Painting or Study Knights Death w/Angel (oil/tempera on cardboard) 28.5" x17"

Hello,This is my Mucha painting,it could be nick-named "Death of a Knight" I suppose.I'm looking at having this both authenticated & appraised but I figured I'd put this up in the meantime. I believe this to be a true Mucha painting.He was known as the "Master of Art Nouveau" and is mainly known for his Posters he created in the late 1800's to early 1900's.He also did a lot of book illustrations & mural paintings.This piece seems to be either a stand-alone painting he never finished or a study for a bigger,better version.I've gone through my 3 Mucha books & researched everything I can find online,but can't find this piece anywhere. I've been analyzing & comparing everything about it for 3 weeks now & can't rule out it being authentic.
It features an Angel,possibly Michael,hovering over the tomb of a knight with the knights wife & child mourning.You can just barely make out lots of detail that was never finished in most of the painting.There seem to be streams/ribbons of flowers & the Angel's scroll/list seems to wrap around through the picture.There are a few wreaths you can just make out,one of them is around the knight's head.One thing that I didn't realize at first is that the gray areas in the borders & in the painting are actually silver paint that was brushed onto the board after the pencil lines were drawn(it's very reflective with direct light).The painting was done on top of the silver layer.Any info would be greatly appreciated.Thanks