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A note....Devise a way to reach-in and scrub-out the box good. If these strops are inserted the wrong way, the paste will be applied to the other side. As you clean out the box, insert the strop...

CroOx is always geen, AFAIK. Either it was pasted with FeOx, (Which is red and finer than CroOx) or some other strop paste, I don't know.
Start with a damp cloth to remove, smooth.
If you get it...