I'm sure I'm not the first person to ever think of this but in 20yrs of loading black powder I've never heard anyone mention it. But I've heard lots of complaints about spilling powder on the floor. So I thought I would share
My goal was to end up with a case full of powder with various compression. (The gun seems to shoot better as the column gets closer to the bullet base). I was goofing around filling the case then compressing a touch, then adding a few grains more then compressing, then putting a wad on....painful!

Then the idea occurred to put a simple extension on the case mouth. I checked the compression die and sure enough there was lots of room for a slightly larger OD collar. 5 minutes later I had whipped up a handy case extension. 

Now its a simple matter to fill the case over full by the amount of compression you wish, drop a wad on top and compress the wad to the case mouth.

Yes what I'm up to here seems a bit unusual but I'm just following the wandering path of what this rifle seems to be telling me. 
It likes the charge column close to the base of the bullet

It does not like thick wads or wad stacks. 

So in order to in order to test the effect of various compression and still end up with a full column this is where I'm at. 

The Scottish in me would be much happier with a 65gr charge but I can't convince the rifle of my economic point of view

I've heard of a few people doing this but have never seen pictures of the implement before.

I think it's a nice idea, when shooting black powder, if you want to experiment with compression when breech seating, or shooting bore diameter PP bullets.   

Thanks for posting the picture.

Chris.