Does That Yellow In Those Mounts Mean That The Stamp Is Damaged / Stained? (Pics Included)

Hard to tell what it is exactly, but yes, it's damage. I expect humidity may have had a lot to do with it.

It looks as though the tape adhesive has migrated into the paper of the mint stamps (second pic).

It is possible that that this hasn't happened to the parcel post stamps. I would remove them from the crystal mounts to see if they are salvageable.

Someone put old fashioned Scotch Tape on the outside edges of the mounts. If you are VERY lucky, none of the stamps have been damaged. If you are very unlucky, ALL of the stamps HAVE been damaged.

I have seen this a lot. I think the original collector felt that the Crystal Mounts that he used were a little 'loose', and he found that the stamps didn't stay put very well. (At least that is the rationale that I have made up in my head) The collector was looking to prevent the stamps from falling out of the side of the mounts, so he put a little piece of Scotch Tape at the open ends of the mounts. It looks great for a few years, then after some time (like decades, I think) the Scotch tape yellows with age. The yellowing will eventually seep through the mount itself and end up staining the stamp inside. At this point, it is like Revcollector says - whether the stamps inside are damaged or not mostly depends on luck. Well, it more depends on whether they have been there long enough for the staining to seep through the mount and into the stamp, but since you probably don't know exactlyhow long the stamps have been in the mounts, it essentially comes down to luck.

I have rolled the dice a number of times on lots stained like this. I have lost that bet enough so that I no longer roll the dice on them. This practice (using Scotch Tape this way) was a big mistake and it became apparent only after decades. I hope nobody today is doing this. Don't give in to the temptation of using Scotch Tape in your stamp albums!

I remember how the tape would damage the mount AND the stamp.

Alas, I have seen a lot of this too. I would expect that at least 60% to 75% are damaged.

What I find rather surprising is that you can still find Crystal Mounts for sale on ebay. Fortunately, I lost only a few common stamps to these mounts many years ago.

Anytime that you see similar discoloration, it is a red flag that the (original) owner was not careful. I may mean that there are other problems with other stamps. Best to stay away from these unless you can view them in person. An ebay seller could easily say that the damage in the photo is not a reason for a return.

There are numerous threads on this forum regarding the woes of Crystal Mount damage. I posted info and some photos of the damage caused in my Dad's collection in this thread:

https://goscf.com/t/57305

Most of his MNH collection has ended up in the garbage. $1,000s down the drain.