Conservation Question +1 1846 GB Cover

I apologize, for this is a cover + stamp question.

1) When I tried opening the cover, there were lots of black specks that I believe is mildew on the inside. Does anyone have a way to handle/treat a cover so that the "growing things" are neutralized without harming the paper too badly?

2) What are the arrows pointing to on the SG8? There are some thin rectangular elevations that resemble embossings in those areas of the stamp that the scanner does not pick up. Or maybe it's pertaining to the incomplete design elements?
Thank you

The arrows are pointing to the frame of the stamp. The frame is very thin above the P of POSTAGE and has some weaknesses down the left side. These are the things that help platers.

As for the black specks, open out the letter gently and blow the dust off. Someone might confirm that the black material is inactive mildew or maybe it's something else?

Hope this info helps

To my eyes it's Plate 61. Would suit that date anyway.

Helps a lot Bobby de la Rue! I just read in a separate thread that microwaving them for a few seconds will silence any living creatures.
I will have to try that!