I got my extra mainspring in last week, and my JBMR finally made it in today. I took it as far down as I could (the nipple is seized, going to get my Kroil and let it soak) - the mainspring is very thick and hefty on this gun. Looking at the two springs, the replacement is identical to the original down to a pressed indention on the side.
I fired two caps, and the channel is clear as it will blow a pile of leaves on the ground away. There was a little more corrosion around the drum than the picture shows but I spent the afternoon polishing up the brass, taking the lock apart and cleaning/lubricating it, and going through everything. I have two stainless 1/4-28 nipples on the way.