Will,
I have that same book, Jentz describes (either in that book or another he wrote) it was a matter of last in, first out, so when the new Chin mantlets came in they got stacked in front of the rounded mantlets, so they were used first as the easiest to get to, then once they were used up they would continue to use the rounded mantlets again until more Chin mantlets came in to block them again.
Anyone who's been around the automotive industry or dealt with inventory has probably experienced this phenomena, with tank parts it is a bit of a problem to rotate the stock to make sure the older parts don't get stale. After all who wants to move around 20 or 50 two ton mantlets just to make sure they get used first.
Your build looks great so far, keep it up. Where did you get the smaller idler wheels and the rounded mantle?
Steve